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3 



BLAKISTON'S ? QUIZ-CO M PEN D[S ? 

A COMPEND 

OF 

HUMAN PHYSIOLOGY 

ESPECIALLY ADAPTED FOR THE USE 
OF MEDICAL STUDENTS 



BY 

ALBERT P. BRUBAKER, A. M., M. D. 

AUTHOR OF "A TEXT-BOOK OF PHYSIOLOGY;" PROFESSOR OF PHYSIOLOGY AND 

MEDICAL JURISPRUDENCE IN THE JEFFERSON MEDICAL COLLEGE; FORMERLY 

PROFESSOR OF PHYSIOLOGY IN THE PENNSYLVANIA COLLEGE OF DENTAL 

SURGERY; FORMERLY LECTURER ON ANATOMY AND PHYSIOLOGY IN 

THE DREXEL INSTITUTE OF ART, SCIENCE, AND INDUSTRY; FELLOW 

OF THE COLLEGE OF PHYSICIANS OF PHILADELPHIA 



FOURTEENTH EDITION 
WITH 26 ILLUSTRATIONS 



PHILADELPHIA 

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Entered according to Act of Congress, in the year 1917, by 

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In the Office of the Librarian of Congress, at Washington, D. C. 




APR 17 1917 



TEE HAPLE PRESS X O K K PA 

©CU460334 



- - 



- PREFACE TO THE FOURTEENTH EDITION 



In the preparation of a new edition of the Compend, an effort has 
been made to increase its usefulness to students of medicine, not only 
by revising and rewriting a large portion of the text, but by 
incorporating a very considerable quantity of new material. This 
it is believed will be helpful to and appreciated by the student body 
for whom the Compend is primarily intended. The incorporation of 
new material has necessitated the elimination of certain portions of 
the text which seemed of relatively less importance to the student 
of physiology, than the portions that have been added. For this 
reason at least, it is believed, that the value of the Compend in its 
present form, is not only increased, but that it will be still more 
acceptable and helpful to the student during his attendance on 
lectures. 

ALBERT P. BRUBAKER. 



CONTENTS 



Page 

Introduction i 

Physiology of the Cell 5 

Physiology of the Skeleton 9 

General Physiology of Muscular Tissue ........ 14 

Special Physiology of Muscles 28 

Physiology of Xerye Tissue 34 

Foods and Dietetics 52 

Digestion 61 

Absorption 86 

Blood 94 

Circulation of the Blood 102 

Respiration 122 

Animal Heat . . . 135 

Excretion 139 

External Secretions 151 

Internal Secretions 161 

The Central Organs of the Nerve System 169 

Spinal Cord 171 

Medulla Oblongata 181 

Pons Varolii 182 

Crura Cerebri 183 

Corpora Quadrigemina 184 

Corpora Striata and Optic Thalami 185 

Cerebrum 188 

Cerebellum 200 

Autonomic Nerve System 202 

Cranial Nerves 206 

N5E of Touch 219 

Sense of Taste 221 

\se of Smell 222 

Sense of Sight 223 

vii 



Vlll CONTENTS 

Pagb 

Sense of Hearing 234 

Phonation and Articulate Speech 243 

Reproduction 248 

Index 259 



A COMPEND 

OF 

HUMAN PHYSIOLOGY 



Introduction. — An animal organism in the living condition exhibits a 
series of phenomena which relate to growth, movement, mentality, and re- 
production. During the period preceding birth, as well as during the 
period included between birth and adult life, the individual grows in size 
and complexity from the introduction and assimilation of material from 
without. Throughout its life the animal exhibits a series of movements, 
in virtue of which it not only changes the relation of one part of its body 
to another, but also changes its position in space. If, in the execution of 
these movements, the parts are directed to the overcoming of opposing 
forces, such as gravity, friction, cohesion, elasticity, etc., the animal may 
be said to be doing work. The result of normal growth is the attainment 
of a physical development that will enable the animal, and, more especially, 
man, to perform the work necessitated by the nature of its environment 
and the character of its organization. In man, and probably in lower 
animals as well, mentality manifests itself as intellect, feeling, and volition. 
At a definite period in the life of the animal it reproduces itself, in con- 
sequence of which the species to which it belongs is perpetuated. 

The study of the phenomena of growth, movement, mentality, and re- 
production constitutes the science of Animal Physiology. But as these 
general activities are the resultant of and dependent on the special activi- 
ties of the individual structures of which an animal body is composed, 
Physiology in its more restricted and generally accepted sense is the 
science which investigates the actions or functions of the individual 
organs and tissues of the body and the physical and chemic conditions 
which underlie and determine them. 



2 HUMAN PHYSIOLOGY 

This may naturally be divided into: 

i. Individual physiology, the object of which is a study of the vital 
phenomena or functions exhibited by the organs of any individual animal. 

2. Comparative physiology, the object of which is a comparison of the 
vital phenomena or functions exhibited by the organs of two or more ani- 
mals, with a view of unfolding their points of resemblance or dissimilarity. 

Human physiology is that department of physiologic science which has 
for its object the study of the functions of the organs of the human body in 
a state of health. 

If the body of any animal be dissected, it will be found to be composed of 
a number of well-defined structures, such as heart, lungs, stomach, brain, 
eye, etc., to which the term organ was originally applied, for the reason 
that they were supposed to be instruments capable of performing some 
important act or function in the general activities of the body. Though 
the term organ is usually employed to designate the larger and more 
familiar structures just mentioned, it is equally applicable to a large 
number of other structures which, though possibly less obvious, are 
equally important in maintaining the life of the individual — e.g., bones, 
muscles, nerves, skin, teeth, glands, blood-vessels, etc. Indeed, any com- 
plexly organized structure capable of performing some function may be 
described as an organ. A description of the various organs which make 
up the body of an animal, their external form, their internal arrangement, 
their relations to one another, constitutes the science of Animal Anatomy. 

This may naturally be divided into: 

i. Individual anatomy, the object of which is the investigation of the 
construction, form, and arrangement of the organs of any individual 
animal. 

2. Comparative anatomy, the object of which is a comparison of the 
organs of two or more animals, with a view of determining their points of 
resemblance or dissimilarity. 

If the organs, however, are subjected to a further analysis, they can be 
resolved into simple structures, apparently homogeneous, to which the 
name tissue has been given — e.g., epithelial, connective, muscle, and nerve 
tissue. When the tissues are subjected to microscopic analysis, it is found 
that they are not homogeneous in structure, but composed of still simpler 
elements, termed cells and fibers. The investigation of the internal struc- 
ture of the organs, the physical properties and structure of the tissues, as 
well as the structure of their component elements, the cells and fibers, con- 
stitutes a department of anatomic science known as Histology, or as it is 
prosecuted largely with the microscope, Microscopic Anatomy. 



PHYSIOLOGIC APPARATUS 3 

Human anatomy is the department of anatomic science which has for its 
object the investigation of the construction of the human body. 

Inasmuch as the study of function or physiology is associated with and 
dependent on a knowledge of structure, it is essential that the student 
should have a general acquaintance with the anatomy and histology not 
only of man but of typical forms of lower animal life as well. Moreover, 
since it has been demonstrated that every exhibition of functional activity 
is associated with changes in the composition of the structures, it has been 
apparent that a knowledge of the chemical composition of the body, as 
well as the successive changes in composition which it undergoes when in 
a state of functional activity, is necessary to a correct understanding of 
the intimate nature of physiologic processes. 

Anatomic Systems. — All the organs of the body which have certain 
peculiarities of structure in common are classified by anatomists into sys- 
tems — e.g., the bones, collectively, constitute the bony or osseous system; 
the muscles, the nerves, the skin, constitute, respectively, the muscular, 
the nervous, and the tegumentary system. 

Physiologic Apparatus. — More important from a physiologic point of 
view than a classification of organs based on similarities of structure is the 
natural association of two or more organs acting together for the accom- 
plishment of some definite object, and to which the term physiologic 
apparatus has been applied. While in the community of organs which 
together constitute the animal body each one performs some definite func- 
tion, and the harmonious cooperation of all is necessary to the life of the in- 
dividual, everywhere it is found that two or more organs, though perform- 
ing totally distinct functions, are cooperating for the accomplishment of 
some larger or compound function in which their individual functions are 
blended — e.g., the mouth, stomach, and intestines, with the glands con- 
nected with them, constitute the digestive apparatus, the object or function 
of which is the complete digestion of the food. The capillary blood-vessels 
and lymphatic vessels of the body, and especially those in relation to the 
villi of the small intestine, constitute the absorptive apparatus, the function 
of which is the introduction of new material into the blood. The heart 
and blood-vessels constitute the circulatory apparatus, the function of 
which is the distribution of blood to all portions of the body. The lungs 
and trachea, together with the diaphragm and the walls of the chest, con- 
stitute the respiratory apparatus, the function of which is the introduction 
of oxygen into the blood and the elimination from it of carbon dioxid 
and other injurious products. The kidneys, the ureters, and the bladder 
constitute the urinary apparatus. The skin, with its sweat-glands, con- 



4 HUMAN PHYSIOLOGY 

stitutes the perspiratory apparatus, the functions of both being the excre- 
tion of waste products from the body. The liver, the pancreas, the mam- 
mary glands, as well as other glands, each form a secretory apparatus which 
elaborates some specific material necessary to the nutrition of the in- 
dividual. The functions of these different physiologic apparatus — e.g., 
digestion, absorption of food, elaboration of blood, circulation of blood, 
respiration, production of heat, secretion, and excretion — are classified as 
nutritive functions, and have for their final object the preservation of the 
individual. 

The nerves and muscles constitute the nervo-muscular apparatus, the 
function of which is the production of motion. The eye, the ear, the nose, 
the tongue, and the skin, with their related structures, constitute, respec- 
tively, the visual, auditory, olfactory, gustatory, and tactile apparatus, the 
function of which, as a whole, is the reception of impressions and the trans- 
mission of nerve impulses to the brain, where they give rise to visual, 
auditory, olfactory, gustatory, and tactile sensations. 

The brain, in association with the sense organs, forms an apparatus 
related to mental processes. The larynx and its accessory organs — the 
lungs, trachea, respiratory muscles, the mouth and resonant cavities of 
the face — form the vocal and articulating apparatus, by means of which 
voice and articulate speech are produced. The functions exhibited by 
the apparatus just mentioned — viz., motion, sensation, language, mental 
and moral manifestations — are classified as functions of relation, as they 
serve to bring the individual into conscious relationship with the external 
world. 

The ovaries and the testes are the essential reproductive organs, the 
former producing the germ-cell, the latter the spermatic element; together 
with their related structures — the fallopian tubes, uterus, and vagina in the 
female, and the urogenital canal in the male — they constitute the repro- 
ductive apparatus characteristic of the two sexes. Their cooperation 
results in the union of the germ-cell and spermatic element and the conse- 
quent development of a new being. The function of reproduction serves 
to perpetuate the species to which the individual belongs. 

The animal body is therefore not a homogeneous organism, but one 
composed of a large number of widely dissimilar but related organs. But 
as all vertebrate animals have the same general plan of organization, there 
is a marked similarity both in form and structure among corresponding 
parts of different animals. Hence it is that in the study of human anat- 
omy a knowledge of the form, construction, and arrangement of the organs 
in different types of animal life is essential to its correct interpretation; also 
it is that in the investigation and comprehension of the complex problems 



PHYSIOLOGY OF THE CELL 5 

of human physiology a knowledge of the functions of the organs as they 
manifest themselves in the different types of animal life is indispensable. 
As many of the functions of the human body are not only complex, but 
the organs exhibiting them are practically inaccessible to investigation, wc 
must supplement our knowledge and judge of their functions by analogy, 
by attributing to them, within certain limits, the functions revealed by 
experimentation upon the corresponding but simpler organs of lower ani- 
mals. This experimental knowledge corrected by a study of the clinical 
phenomena of disease and the results of post-mortem investigations, forms 
the basis of modern human physiology. 

PHYSIOLOGY OF THE CELL 

A histologic analysis of the tissues shows that they can be resolved into 
simpler elements, termed cells, which may, therefore, be regarded as the 
primary units of structure. Though cells vary considerably in shape, size, 
and chemic composition in the different tissues of the adult body, they are 
nevertheless, descendants from typical cells, known as embryonic or 
undifferentiated cells, the first offspring of the fertilized ovum. Ascend- 
ing the line of embryonic development, it wall be found that every organ- 
ized body originates in a single cell — the ovum. As the cell is the elemen- 
tary unit of all tissues, the function of each tissue must be referred to the 
function of the cell. Hence the cell may be defined as the primary 
anatomic and physiologic unit of the organic world, to which every exhibi- 
tion of life, whether normal or abnormal, is to be referred. 

Structure of Cells. — Though cells vary in shape and size and internal 
structure in different portions of the body, a typical cell may be said to 
consist mainly of a gelatinous substance forming the body of the cell, 
termed cytoplasm or bioplasm, in which is embedded a smaller spheric body, 
the nucleus. Within the nucleus there is frequently a still smaller body 
the nucleolus. The shape of the adult cell varies according to the tissue 
in which it is found; when young and free to move in a fluid medium, the 
cell assumes a spheric form, but when subjected to pressure, may become 
cylindric, fusiform, polygonal, or stellate. Cells vary in size within wide 
limits, ranging from M 200 of an inch, the diameter of a red blood-corpuscle, 
to Moo of an inch, the diameter of the large cells in the gray matter of the 
spinal cord. 

The cell cytoplasm ( onsistfl of a soft, semifluid, gelatinous material, vary 
ing somewhat in appearance in different tissues. Though frequently 
homogeneous, it often exhibits a finely granular appearance under medium 



6 HUMAN PHYSIOLOGY 

powers of the microscope. Young cells consists almost entirely of clear 
cytoplasm, mature cells contain, according to the tissue in which they are 
found, material of an entirely different character — e.g., small globules of 
fat, granules of glycogen, mucigen, pigments, digestive ferments, etc. 
Under high powers of the microscope the cytoplasm is found to be per- 
vaded by a network of fibers, termed spongioplasm, in the meshes of which 
is contained a clearer and more fluent substance, the hyaloplasm. The 
relative amount of these two constituents varies in different cells, the 
proportion of hyaloplasm being usually greater in young cells. The 
arrangement of the fibers forming the spongioplasm also varies, the fibers 
having sometimes a radial direction, in others a concentric disposition, 
but most frequently being distributed evenly in all directions. In many 
cells the outer portion of the cell cytoplasm undergoes chemic changes 
and is transformed into a thin, transparent, homogenous membrane — the 
cell membrane — which completely incloses the cell substance. The cell 
membrane is permeable to water and watery solutions of various inorganic 
and organic substances. It is, however, not an essential part of the cell. 

The nucleus is a small vesicular body embedded in the cytoplasm near 
the center of the cell. In the resting condition of the cell it consists of a 
distinct membrane, composed of amphipyrenin, inclosing the nuclear 
contents. The latter consists of a homogenous amorphous substance — the 
nuclear matrix — in which is embedded the nuclear network. It can often 
be seen that a portion of one side of the nucleus, called the pole, is free 
from this network. The main cords of the network are arranged as V- 
shaped loops about it. These main cords send out secondary branches or 
twigs, which, uniting with one another, complete the network. The nu- 
clear cords are composed of granules of chromatin — so called because of its 
affinity for certain staining materials — held together by an achromatin 
substance known as linen. Besides the nuclear network, there are em- 
bedded in the nuclear matrix one or more small bodies composed of pyrenin 
known as nucleoli. At the pole of the nucleus, either within or just with- 
out in the cytoplasm, is a small body, the centrosome, or pole corpuscle. 

Chemic Composition of the Cell. — The composition of living protoplasm 
is difficult of determination, for the reason that all chemic and physical 
methods employed for its analysis destroy its vitality, and the products 
obtained are peculiar to dead rather than to living matter. Moreover, 
as protoplasm is the seat of constructive and destructive processes, it is not 
easy to determine whether the products of analysis are crude food constitu- 
ents or cleavage or disintegration products. Nevertheless, chemic inves- 
tigations have shown that even in the living condition protoplasm is a 



PHYSIOLOGY OF THE CELL 7 

highly complex compound — the resultant of the intimate union of many 
different substances. About seventy-five per cent, of protoplasm consists 
of water and twenty-five per cent, of solids, of which the more important 
compounds are various nucleo-proteins (characterized by their large 
percentage of phosphorus), globulins, traces of lecithin, cholesterin and 
frequently fat and carbohydrates. Inorganic salts, especially the potas- 
sium, sodium, and calcium chlorids and phosphates, are almost invariable 
and essential constituents. 

MANIFESTATIONS OF CELL LIFE 

Growth, the Maintenance of Nutrition, and Reproduction. — All cells 
exhibit three fundamental properties of life — viz., growth, the main- 
tenance of their nutrition, and reproduction. Growth is an increase in 
size. When newly reproduced all cells are extremely small, but in conse- 
quence of their organization and the character of their surrounding 
medium, they gradually grow until they attain the size characteristic 
of the adult state. 

Nutrition may be defined as the sum of the processes concerned in the 
maintenance of the physiologic condition of the cell and includes both 
growth and repair. So long as this is accomplished, the cells and the 
tissues which are formed by them continue to exhibit their functions or 
their characteristic modes of activity. Both growth and nutrition are 
dependent on the power which living material possesses of not only 
absorbing nutritive material from the surrounding medium, the lymph, 
but of subsequently assimilating it, organizing it, transforming it into 
material like itself and endowing it with its own physiologic properties. 

In the physiologic condition the living material of the cell, the bio- 
plasm, is the seat of a series of chemic changes which vary in degree from 
moment to moment in accordance with the degree of functional activity, 
and on the continuance of which all life phenomena depend. Some of 
these chemic changes are related to or connected with the molecules of 
the living material, while others are connected with the food material 
supplied to them. Of the chemic changes occurring within the mole- 
cules some are destructive, dissimilative or disintegrative in character, 
whereby the molecule is in part eventually reduced through a series of 
descending chemic stages to simpler compounds which, apparently of 
no use in the cell, are eliminated from it. It is, therefore, said that the 
living material undergoes molecular disintegration as a result of func- 
tional activity. To these changes the term katabolism is also applied. 
Other of these changes are constructive, assimilative or integrative in 



8 HUMAN PHYSIOLOGY 

character,, whereby a part at least of the food material furnished by the 
blood-plasma is transformed through a series of ascending chemic stages 
into living material, and whereby it is repaired and its former physio- 
logic condition restored. It is, therefore, said that the living material 
undergoes molecular integration as a preparation for functional activity. 
To these changes the term anabolism is also applied. During the course 
of its physiologic activities the cell bioplasm produces materials of an 
entirely different character which vary with the cell, such as fat, glycogen, 
mucigen, pigments, ferments, etc., which are generally spoken of as 
metabolic products. 

Living material has also a temperature varying in degree in different 
species of animals as well as in different parts of the same animal. Here 
as elsewhere the temperature is due to heat liberated from organic com- 
pounds through disruption and subsequent oxidation to simpler com- 
pounds. Though some of the heat liberated may come from the tissue 
molecules, the larger part by far comes from the food molecules — sugar, 
fat, and protein, constituents of the fluids circulating in the tissue spaces. 
These foods carry into the body potential energy, ultimately derived 
from the sun. When they are disrupted and oxidized the potential 
energy is transformed into kinetic energy which manifests itself for the 
most part as heat. To the sum total of all the chemic changes occurring 
in tissues and foods the term metabolism is given. 

Physiologic Properties of Protoplasm. — All living protoplasm possesses 
properties which serve to distinguish and characterize it — viz., irritability, 
conductivity, and motility. 

Irritability, or the power of reacting in a definite manner to some form 
of external excitation, whether mechanical, chemic, or electric, is a funda- 
mental property of all living protoplasm. The character and extent of 
the reaction will vary, and will depend both on the nature of the proto- 
plasm and the character and strength of the stimulus. If the protoplasm 
be muscle, the response will be a contraction; if it be gland, the response 
will be secretion; if it be nerve, the response will be a sensation or some 
other form of nerve activity. 

Conductivity, or the power of transmitting molecular disturbances 
arising at one point to all portions of the irritable material, is also a char- 
acteristic feature of all protoplasm. This power, however, is best de- 
veloped in that form of protoplasm found in nerves, which serves to 
transmit, with extreme rapidity, molecular disturbances arising at the 
periphery to the brain, as well as in the reverse direction. Muscle 
protoplasm also possesses the same power in a high degree. 



THE PHYSIOLOGY OF THE SKELETON 9 

Motility, or the power of executing apparently spontaneous move- 
ments, is exhibited by many forms of cell protoplasm. In addition to 
the molecular movements which take place in certain cells, other forms 
of movement are exhibited, more or less constantly, by many cells in 
the animal body — e.g., the waving of cilia, the ameboid movements and 
migrations of white blood corpuscles, the activities of spermatozooids, 
the projections of pseudopodia, etc. These movements, arising without 
any recognizable cause, are frequently spoken of as spontaneous. Strictly 
speaking, however, all protoplasmic movement is the resultant of natural 
causes, the true nature of which is beyond the reach of present methods 
of investigation. 

Reproduction. — Cells reproduce themselves in the higher animals in 
two ways — by direct division and by indirect division, or karyokinesis. 
In the former the nucleus becomes constricted, and divides without any 
special grouping of the nuclear elements. It is probable that this occurs 
only in disintegrating cells, and never in a physiologic multiplication. 
In division by karyokinesis there is a progressive rearranging and definite 
grouping of the nucleus, the result of which changes is the division of 
the centrosome, the chromatin, and the rest of the nucleus into two 
equal portions, which form the nuclei. Following the division of the 
nuclei, the protoplasm becomes constricted midway between the young 
nuclei. This constriction gradually deepens until the original cell is 
divided, with the formation of two complete cells. 






THE PHYSIOLOGY OF THE SKELETON 



The animal body is characterized by the power of executing a great 
variety of movements, all of which have reference to a change of relation 
of one part of the body to another, or to a change of position of the indivi- 
dual in its environment, as in the various acts of locomotion. If in the 
execution of these movements the different parts are applied or directed 
to the overcoming of opposing forces in the environment, the animal is said 
to be doing work. In the conception of the animal body as a machine 
for the accomplishment of work the skeleton, the muscle and nerve tissues, 
constitute the three primary mechanisms, all of which bear certain defi- 
nite relations one to another. 

The skeleton in its entirety determines the plan of organization of the 

animal body and imparts to it its characteristic features. In its entirety 

. es for the attachment of muscles, the support of viscera and by 

reason of the relation of the bones one to another, permits of a great variety 



IO HUMAN PHYSIOLOGY 

of movements. The skeleton may be divided into an axial and an appen- 
dicular portion. 

The Axial Portion. — The axial portion consists of the bones of the head, 
of the vertebral column and the ribs. The vertebral column is the founda- 
tion element and the center around which the appendicular portions are 
developed and arranged with a certain degree of conformity. It is com- 
posed of a series of superimposed bones, termed vertebrae, which increase in 
size from above downward as far as the brim of the pelvic cavity. Supe- 
riorly, it supports the skull; laterally, it affords attachment for the ribs, 
which in turn support the weight of the upper extremities; below, it rests 
upon the pelvic bones, which transmit the weight of the body to the in- 
ferior extremities. The bodies of the vertebrae are united one to another 
by tough elastic discs of fibro-cartilage, which, collectively, constitute 
about one-quarter of the length of the vertebral column. The vertebrae are 
held together by ligaments situated on the anterior and posterior surfaces 
of their bodies, and by short, elastic ligaments between the neural arches 
and processes. These structures combine to render the vertebral column 
elastic and flexible, and enable it to resist and diminish the force of shocks 
communicated to it. The character and the arrangement of the bones of 
the axial portion endow the animal mechanism with a certain degree of 
fixity combined with slight mobility. 

The Appendicular Portion. — The appendicular portion consists of the 
bones of the arms and legs, the scapular and pelvic arches. By reason of 
its character and anatomic arrangement, the animal body is endowed 
with extreme mobility, enabling the animal to execute a great variety of 
rapid and extensive movements which, however, vary in degree in different 
animals in accordance with their organization and the nature of their 
environment. 

For the manifestation of the activities of the animal it is essential that 
the relation of the various portions of the bony skeleton to one another 
shall be such as to permit of movement while yet retaining close apposi- 
tion. This is accomplished by the mechanical conditions which have been 
evolved at the points of union of bones, and which are technically known 
as articulations or joints. 

A consideration of the body movements involves an account of (i) the 
static conditions, or those states of equilibrium in which the body is at 
rest — e.g., standing, sitting; (2) the dynamic conditions, or those states of 
activity characterized by movement — e.g., walking, running, etc. In this 
connection, however, only those physical and physiologic peculiarities of 



THE PHYSIOLOGY OF THE SKELETON II 

the skeleton, especially in its relation to joints, will be referred to which 
underlie and determine both the static and dynamic states of the body. 

Structure of Joints. — The structures entering into the formation of 
joints are: 

i. Bones, the articulating surfaces of which are often more or less ex- 
panded, especially in the case of long bones, and at the same time variously 
modified and adapted to one another in accordance with the character and 
extent of the movements which there take place. 

2. Hyaline cartilage, which is closely applied to the articulating end of 
each bone. The smoothness of this form of cartilage facilitates the move- 
ments of the opposing surfaces, while its elasticity diminishes the force of 
shocks and jars imparted to the bones during various muscular acts. In 
a number of joints, plates or discs of white fibro-cartilage are inserted 
between the surfaces of the bones. 

3. A synovial membrane, which is attached to the edge of the hyaline 
cartilage entirely inclosing the cavity of the joint. This membrane is 
composed largely of connective tissue, the inner surface of which is lined 
by endothelial cells, which secrete a clear, colorless, viscid fluid — the 
synovia. This fluid not only fills up the joint-cavity, but, flowing over the 
articulating surfaces, diminishes or prevents friction. 

4. Ligaments — tough, inelastic bands, composed of white fibrous tissue 
— which pass from bone to bone in various directions on the different 
aspects of the joint. As white fibrous tissue is inextensible but pliant, 
ligaments assist in keeping the bones in apposition, and prevent displace- 
ment while yet permitting of free and easy movements. 

Classification of Joints. — All joints may be divided, according to the 
extent and kind of movements permitted by them, into (1) diarthroses; 
(2) amphiarthroscs; (3) synarthroses. 

A. Diarthroses. — In this division of the joints are included all those 
which permit of free movement. In the majority of instances the articu- 
lating surfaces are mutually adapted to each other. If the articulating 
surface of one bone is convex, the opposing but corresponding surface is 
concave. Each surface, therefore, represents a section of a sphere or 
cylinder, which latter arises by rotation of a line around an axis in space. 
According to the number of axes around which the movements take place 
all diarthrodial joints may be divided into: 

1. Uniaxial Joints. — In this group the convex articulating surface is a 
segment of a cylinder or cone, to which the opposing surface more or less 



12 HUMAN PHYSIOLOGY 

completely corresponds. In such a joint the single axis of rotation, 
though, practically is not exactly at right angles to the long axis of the 
bone, and hence the movements — flexion and extension — which take place 
are not confined to one plane. Joints of this character — e.g., the elbow, 
knee, ankle, the phalangeal joints of the fingers and toes — are, therefore, 
termed ginglymi, or hinge-joints. Owing to the obliquity of their articu- 
lating surfaces, the elbow and ankle are cochleoid or screw- ginglymi. Inas- 
much as the axes of these joints on the opposite sides of the body are not 
coincident, the right elbow and left ankle are right-handed screws; the left 
elbow and right ankle, left-handed screws. In the knee-joint the form 
and arrangement of the articulating surfaces are such as to produce that 
modification of a simple hinge known as a spiral hinge, or helicoid. As 
the articulating surfaces of the condyles of the femur increase in convexity 
from before backward, and as the inner condyle is longer than the outer, 
and therefore, represents a spiral surface, the line of translation or the 
movement of the leg is also a spiral movement. During flexion of the leg 
there is a simultaneous inward rotation around a vertical axis passing 
through the outer condyle of the femur; during extension a reverse move- 
ment takes place. Moreover, the slightly concave articulating surfaces 
of the tibia do not revolve around a single fixed transverse axis, as in the 
elbow-joint, for during flexion they slide backward, during extension 
forward, around a shifting axis, which varies in position with the point of 
contact. 

In some few instances the long axis of the articulating surface is parallel 
rather than transverse to the long axis, and as the movement then takes 
place around a more or less conic surface, the joint is termed a trochoid 
or pulley — e.g., the odonto-atlantal and the radio-ulnar. In the former 
the collar formed by the atlas and its transverse ligaments rotates around 
the vertical odontoid process of the axis. In the latter the head of the 
radius revolves around its own long axis upon the ulna, giving rise to the 
movements of pronotion and supination of the hand. The axis around 
which these two movements take place is continued through the head of 
the radius of the styloid process of the ulna. 

2. Biaxial Joints. — In this group the articulating surfaces are unequally 
curved, though intersecting each other. When the surfaces lie in the 
same direction, the joint is termed an ovoid joint — e.g., the radio-carpal 
and the atlanto-occipital. As the axes of these surfaces are vertical to 
each other, the movements permitted by the former joint are flexion, 
extension, adduction, and abduction, combined with a slight amount of 
circumduction; the latter joint permits of flexion and extension of the 



THE PHYSIOLOGY OF THE SKELETON 1 3 

head, with inclination to either side. When the surfaces do not take the 
same direction, the joint, from its resemblance to the surfaces of a saddle, 
is termed a saddle-joint — e.g., the trapezio-metacarpal. The movements 
permitted by this joint are also flexion, extension, adduction, abduction, 
and circumduction. 

3. Poly axial Joints. — In this group the convex articulating surface is a 
segment of a sphere, which is received by a socket formed by the oppos- 
ing articulating surface. In such a joint, termed an enarthrodial or ball- 
and-socket joint — e.g., the shoulder- joint, hip- joint — the distal bone 
revolves around an indefinite number of axes, all of which intersect one 
another at the center of rotation. For simplicity, however, the move- 
ment may be described as taking place around axes in the three ordinal 
planes — viz., a transverse, a sagittal, and a vertical axis. The move- 
ments around the transverse axis are termed flexion and extension; 
around the sagittal axis, adduction and abduction; around the vertical 
axis, rotation. "When the bone revolves around the surface of an imaginary 
cone, the apex of which is the center of rotation and the base the curve 
described by the hand, the movement is termed circumduction. 

B. Am phi arthroses. — In this division are included all these joints 
which permit of but slight movement — e.g., the intervertebral, the inter- 
pubic, and the sacro-iliac joints. The surfaces of the opposing bones are 
united and held in position largely by the intervention of a firm, elastic 
disc of fibro-cartilage. Each joint is also strengthened by ligaments. 

C. Synarthroses. — In this division are included all those joints in which 
the opposing surfaces of the bones are immovably united, and hence 
do not permit of any movement — e.g., the joints between the bones of 
the skull. 

Levers. — In the animal machine, as in physical machines generally, 
work is accomplished by the intermediation of levers. The bones col- 
lectively constitute a system of levers the fulcra of which lie in the joints. 
The long bones more especially, are the levers which are employed by the 
muscles to overcome the opposing forces or resistances. The structure 
and the chemic composition of the bones, consisting as they do of inorganic 
matter 67 per cent, and of organic matter 33 per cent, endow them with 
both rigidity and elasticity, physical properties which admirably adapt 
them to the character of the work necessitated by the environment and 
the organization of the animal. 

That a lever may be effective as an instrument for the accomplishment 
of work, it must not only be capable of moving around its fulcrum, but 



14 HUMAN PHYSIOLOGY 

it must at the same time be acted on by two opposing forces, one passive, 
the other active. In the movement of the bony levers of the animal 
body, the passive forces are largely those connected with the environ- 
ment, e.g. y gravity, cohesion, friction, elasticity, etc. The active forces 
by which these latter are opposed and overcome through the intermediation 
of the bony levers are found in the muscles attached to them. 

In all the static and dynamic states of the body the vertebral column 
plays a most essential role. The amphiarthrodial character of the 
intervertebral joint endows the entire column with certain forms of 
movement that are necessary to the performance of many body activities. 

While the range of movement between any two vertebrae is slight, the 
sum total of movement of the entire series of vertebrae is considerable. 
In different regions of the column the character, as well as the range of 
movement, varies in accordance with the forms of the vertebrae and the 
inclination of their articular processes. In the cervical and lumbar regions 
extension and flexion are freely permitted, though the former is greater 
in the cervical, the latter in the lumbar region, especially between the 
fourth and fifth vertebrae. Lateral flexion takes place in all portions of 
the column, but is particularly marked in the cervical region. A rotatory 
movement of the column as a whole takes place through an angle of about 
twenty-eight degrees. This is most evident in the lower cervical and 
dorsal regions. 

The diarthrodial character of the joints of the appendicular portions 
permit of extremely free movements. The character of the movements 
as well as their extent depends largely on the shape and adjustment of 
the bones at their points of union. 

GENERAL PHYSIOLOGY OF MUSCLE TISSUE 

The muscle tissue, which closely invests the bones of the body, and 
which is familiar to all as the flesh of animals, is the immediate cause 
of the active movements of the body. This tissue is grouped in masses 
of varying size and shape, which are technically known as muscles. The 
majority of the muscles of the body are connected with the bones of the 
skeleton in such a manner that, by an alteration in their form, they can 
change not only the position of the bones with reference to one another, 
but can also change the individual's relation to surrounding objects. 
They are, therefore, the active organs of both motion and locomotion, 
in contradistinction to the bones and joints, which are but passive agents 
in the performance of the corresponding movements. In addition to the 
muscle masses which are attached to the skeleton, there are also other 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE IS 

collections of muscle tissue surrounding cavities such as the stomach, 
intestine, blood-vessels, etc., which impart to their walls motility, and 
so influence the passage of a material through them. 

Muscles produce movement of the structures to which they are attached 
by the property with which they are endowed of changing their shape, 
shortening or contracting under the influence of a stimulus transmitted 
to them from the nervous system. Muscles are therefore divided into: 

i. Skeletal muscles, comprising those muscles which are attached to 
the various bones of the skeleton. 

2. Visceral muscles, comprising those muscles which are found in and 
which compose a portion of the walls of the hollow viscera. 

As the skeletal muscles are capable of being excited to activity by nerve 
impulses descending from the cerebrum as a result of volition they are 
frequently termed voluntary muscles. By reason of their appearance as 
seen under the microscope they are termed also striped or striated muscles. 
As the visceral muscles are not capable of being excited to action by voli- 
tion they are frequently termed involuntary muscles. By reason of 
their appearance as seen under the microscope they are termed also 
non-striated or smooth muscles. 

Though for the most part the skeletal muscles are red in color, there are 
certain muscles in man and other animals which are pale in color and in 
many muscles, pale fibers are extensively distributed among the red fibers. 

The Skeletal Muscle. — All skeletal muscles consist of a central fleshy 
portion, the body or belly, which is provided at either extremity with 
a tendon in the form of a cord or membrane by which it is attached to the 
bones. The body is the contractile region, the source of activity; the 
tendon is a passive region, and merely transmits the activity to the 
bones. 

A skeletal muscle is a complex organ consisting of muscular fibers, 
connective tissue, blood-vessels, and lymphatics. The general body of 
the muscle is surrounded by a dense layer of connective-tissue, the 
cpimysium, which blends with and partly forms the tendon; from its 
inner surface septa of connective tissue pass inward and group the muscle- 
fibers into larger and smaller bundles, termed fasciculi. The fasciculi, 
invested by this special sheath, the perimysium, are irregular in shape, 
and vary considerably in size. The fibers of the fasciculi are separated 
from one another and supported by a delicate connective tissue, the 
endomysium. The connective tissue thus surrounding and penetrating 
the muscle binds its fibers into a distinct organ, and affords support to 
blood-vessels, nerves, and lymphatics. The muscle fibers are arranged 



1 6 HUMAN PHYSIOLOGY 

parallel to one another, and their direction is that of the long axis of the 
muscle. In length they vary from thirty to forty millimeters, and in 
diameter from twenty to thirty micromillimeters. 

Histology of the Skeletal Muscle-Fiber. — A muscle-fiber consists of a 
transparent elastic membrane, the sarcolemma, in which is contained the 
true muscle element. Examined microscopically, the fiber presents a 
series of alternate dim and bright bands, giving to it a striated appearance. 

When the bright band is^examined with high magnifying powers, a fine, 
dark line is seen crossing it transversely. It was supposed by Krause to be 
the optic expression of a membrane attached laterally to the sarcolemma. 

The muscle-fiber also exhibits a longitudinal striation, indicating that it 
is composed of fibrillar, placed side by side and embedded in some inter- 
fibrillar substance, to which the name" sarcoplasm has been given. The 
fibrillar, which are arranged longitudinally to the long axis of the fiber, are 
grouped by the intervening material into bundles of varying size, the 
muscle columns. The fibrillar which extend throughout the length of the 
fiber are apparently of uniform thickness, passing directly through the 
transverse membrane and being supported by it. 

In the region of the dim band the fibrilla presents itself in the form of a 
homogeneous prismatic rod, termed sarcostyle, separated from neighboring 
rods by a slight amount of sarcoplasm. 

The Blood Supply. — The blood supply to the muscle is very great, and 
the disposition of the capillary vessels, with reference to muscle-fiber, is 
very characteristic. The arterial vessels, after entering the muscle, are 
supported by the perimysium; in this situation they give off short, trans- 
verse branches, which immediately break up into a capillary network 
of rectangular shape, within which the muscle-fibers are contained. The 
muscle-fiber in intimate relation with the capillary is bathed with lymph 
derived from it. Its contractile substance, however, is separated from 
the lymph by its own investing membrane, through which all interchange 
of nutritive and waste materials must take place. Lymphatics are pre- 
sent in muscle, but are confined to the connective tissue, in the spaces of 
which they have their origin. 

The Nerve Supply. — The nerves which carry the stimuli to a muscle 
enter near its geometric center. Many of the fibers pass directly to the 
muscle-fibers with which they are connected; others are distributed to 
blood-vessels. Every muscle-fiber is supplied with a special nerve-fiber, 
except in those instances where the nerve trunks entering a muscle do not 
contain so many fibers as the muscle. In such cases the nerve-fibers 
divide, until the number of branches equals the number of muscle-fibers. 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE 1 7 

The individual muscle-fiber is penetrated near its center by the nerve, the 
ends being practically free from nerve influence. The stimulus that comes 
to the muscle fiber acts primarily upon its center, and theji travels in both 
directions to the ends. 

Chemic Composition of Muscle. — The chemic composition of muscle, 
is imperfectly understood, owing to the fact that some of its constitu- 
ents undergo a spontaneous coagulation after death, and that the chemic 
methods employed also tend to alter its normal composition. When 
fresh muscle is freed from fat and connective tissue, frozen, rubbed up in 
a mortar, and expressed through linen, a slightly yellow, syrupy, alkaline, 
or neutral fluid is obtained, known as muscle plasma. This fluid at nor- 
mal temperature coagulates spontaneously, and resembles in many re- 
spects the coagulation of blood plasma. The coagulum subsequently 
contracts and squeezes out an acid muscle serum. The coagulated mass 
is termed myosin or myogen fibrin. This protein belongs to. the class of 
globulins!' Inasmuch as it is not present in living muscle, and makes its 
appearance only in the as yet living muscle plasma, it is probable that it 
is derived from some preexisting substance, which is supposed to be 
myosinogen or myogen. Myosin is digested by pepsin and trypsin. 
According to Halliburton, muscle plasma contains the following protein 
bodies: Myosinogen, paramyosinogen, albumin, myoalbumose, all of 
which differ in chemic composition and respond to various chemic and 
physical reagents. 

Ferment bodies, such as pepsin and diastase; non-nitrogenized bodies, 
such as glycogen, lactic and sarcolactic acids, fatty bodies, and inosite; 
nitrogenized extractives — e.g., urea, uric acid, kreatinin, as well as in- 
organic salts, have been obtained from the muscle serum. 

The Physical Properties of Muscle Tissue. — The consistency of muscle 
tissue varies considerably, according to the different states of the muscle. 
In a state of tension it is hard and resistant; when free from tension, it is 
soft and fluctuating, whether the muscle is contracting or resting. Ten- 
sion alone produces hardness. The cohesion of muscle tissue is less than 
that of connective tissue, and is broken more readily. Cohesion resists 
traction and pressure, and lasts as long as irritability remains. 

The elasticity of a muscle, though not great, is almost perfect. After 
being extended by a weight, it returns to its natural form. The limit 
of elasticity, however, is soon passed. A weight of 50 or 100 grams will 
overcome the elasticity so that it will not return to its natural length. In 
inorganic bodies the extension is directly proportional to the extending 
weight, and the line of extension is straight. With muscles, the extension 
2 



1 8 HUMAN PHYSIOLOGY 

is not proportional to the weight. While at first it is marked, the elonga- 
tion diminishes as the weight increases by equal increments, so that the 
line of extension becomes a curve. In other words, the elasticity of a pas- 
sive muscle augments with increased extension. On the contrary the 
elasticity of an active is less than that of a passive muscle, for it is elon- 
gated more by the same weight, as shown by experiment. 

Tonicity is a property of all muscles in the body, in consequence of being 
normally stretched to a slight extent beyond their natural length. This 
may be due to the action of antagonistic muscles, or to the elasticity of the 
parts of the skeleton to which they are attached. This is shown by the 
shortening of the muscle which takes place when it is divided. Muscular 
tonus plays an important r61e in muscular contraction. Being always on 
the stretch, the muscle loses no time in acquiring that degree of tension 
necessary to its immediate action on the bones. Again, the working 
power of a muscle is increased by the presence of some resistance to the 
act of contraction. According to Marey, the amount of work is con- 
siderably increased when the muscular energy is transmitted by an 
elastic body to the mass to be moved, while at the same time, the shock 
of the contraction is lessened. The position of a passive limb is the resul- 
tant also of the elastic tension of antagonistic groups of muscles. 

Muscle excitability and contractility are terms employed to denote that 
property of muscle tissue in virtue of which it contracts or shortens in 
response to various excitants or stimuli. Though usually associated with 
the activity of the nervous system, it is nevertheless an independent en- 
dowment, and persists after all nervous connections are destroyed. If the 
nerve terminals be destroyed, as they can be by the introduction of curara 
into the system, the muscles become completely relaxed and quiescent. 
The strongest stimuli applied to the nerves fail to produce a contraction. 
Various external stimuli applied directly to the muscle substance produce 
at once the characteristic contraction. The excitability of muscle is there- 
fore an inherent property, dependent on its nutrition, and persisting as 
long as it is supplied with proper nutritive materials and surrounded by 
those external conditions which maintain its chemic or physical integrity. 

| Muscle Contractions. — All muscle contractions occurring in the body 
under normal physiologic conditions are either voluntary, caused by a 
volitional effort and the transmission of a nerve impulse from the brain 
through the spinal cord and nerves to the muscles, or reflex, caused by a 
peripheral stimulation and the transmission of a nerve impulse to the 
spinal cord, to be reflected outward through the same nerves to the mus- 
cles. In either case the resulting contraction is essentially the same. The 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE 19 

normal or physiologic stimulus which provokes the muscular contraction 
is a nerve impulse the nature of which is unknown, but is perhaps allied 
to a molecular disturbance. After removal from the body, muscles 
remain in a state of rest, inasmuch as they possess no spontaneity of 
action. Though consisting of a highly irritable tissue, they cannot pass 
from the passive to the active state except upon the application of some 
form of stimulation. 

The stimuli which are capable of calling forth a contraction may be 
divided into: 

1. Mechanical. 2. Chemic. 3. Physical; 4. Electric. 

Every mechanical stimulus of a muscle — e.g., pick, cut, or tap — pro- 
vided it has sufficient intensity, and is repeated with sufficient rapidity, 
will cause not only a single contraction, but a series of contractions. 

All chemic agents which impair the chemic composition of the muscle 
with sufficient rapidity — e.g., hydrochloric acid, acetic and oxalic acids, 
distilled water injected into the vessels, etc. — act as stimuli, and produce 
single and multiple contractions. Physical agents, as heat and elec- 
tricity, also act as stimuli. A muscle heated rapidly to 3o°C. contracts 
vigorously, and reaches its maximum at 45°C. Of all forms of stimuli, 
the electric is the most generally used. Two forms are used — the induced 
current and the make-and-break of a constant current. 

Changes in a Muscle During Contraction. — When a muscle is stimu- 
lated, either indirectly through the nerve or directly by any external 
agent, it undergoes a series of changes, which relate to its form, volume, 
optic, physical chemic, and electric properties. These changes, in their 
totality, constitute the muscular contraction. 

1. Form. — The most obvious change is that of form. The fibers become 
shorter in their longitudinal and wider in their transverse diameters, and 
the muscle as a whole becomes shorter and thicker. The degree of 
shortening may amount to thirty per cent, of the original length. 

2. Volume. — The increase in transverse diameter does not fully com- 
pensate for the diminution in length, for there is at the moment of con- 
traction a slight shrinkage in volume, which has been attributed to a 
compression of air in its interstices. 

3. Optic Changes. — If a muscle-fiber be examined microscopically dur- 
ing its contraction, it will be observed that when the contraction wave 
begins, both bright and dim bands diminish in height and become broader, 
though this change is more noticeable in the region of the bright band. 
This Englemann attributes to a passage of fluid material from the bright 



20 HUMAN PHYSIOLOGY 



into the dim band. At the time of relaxation there is a return of this 
material, and the fiber assumes its original shape and volume. As the 
contraction wave reaches its maximum, the optic properties of both the 
isotropic and anisotropic bands change. The former, which was origi- 
nally clear, now becomes darker and less transparent, until at the crest of 
the wave it assumes the appearance of a distinct dark band. The latter, 
the anisotropic, which was originally dim, now becomes, in comparison, 
clear and light. This change in optic appearance is due to an increase in 
refrangibility of the isotropic and a decrease in the anisotropic bands 
coincident with the passage of fluid from the former into the latter. There 
is at the height of the contraction a complete reversal in the positions of 
the striations. At a certain stage between the beginning and the crest 
of the wave there is an intermediate point, at which the striae almost 
entirely disappear, giving to the fiber an appearance of homogeneity. 
There is, however, no change in refractive power, as shown by the polar- 
izing apparatus. After the contraction wave has reached the stage of 
greatest intensity, there is a reversal of the foregoing phenomena, and 
the fiber returns to its original condition, which is one of relaxation. 

4. Physical Changes. — The extensibility of muscle is increased during the 
contraction, the same weight elongating the fibers to a greater extent 
than during rest. The elasticity, or its power of returning to its original 
form, is correspondingly diminished. 

5. C hemic Changes. — The chemic changes which take place in a muscle 
during contraction or activity are very complex. 

As shown by an analysis of the blood flowing to and from the resting 
muscle, it has, while passing through the capillaries, lost oxygen and 
gained carbon dioxid. The amount of oxygen absorbed by the muscle 
(nine per cent.) is greater than the amount of CO2 given off (6.7 per cent.). 
There is no parallelism between these two processes, as CO2 will be given 
off in the absence of oxygen, or in an atmosphere of nitrogen. 

In the active or contracting muscle both the absorption of oxygen and 
the production of C0 2 are largely increased, but the ratio existing between 
them differs considerably from that of the resting muscle, for the quantity 
of oxygen absorbed amounts to 11.26 per cent., the quantity of C0 2 to 
10.8 per cent. (Ludwig). Moreover, in a tetanized muscle the quantity 
of CO2 given off may be largely in excess of the oxygen absorbed. From 
these facts it is evident that the energy of the contraction does not 
depend upon the direct oxidation of certain substances, but upon the 
decomposition of some unstable compound of high potential energy, 
rich in carbon and oxygen. When the muscle is active, its tissue changes 






GENERAL PHYSIOLOGY OF MUSCLE TISSUE 21 

from a neutral to an acid reaction, from the development of sarcolactic 
and possibly phosphoric acids. The amount of glycogen present in 
muscle (0.43 per cent.) diminishes, but muscles wanting in glycogen, 
nevertheless, retain their power of contraction. Water is absorbed. 
The amount of urea is not materially increased by muscular activity, 
unless it is excessive and prolonged, and then only in the absence of 
a sufficient quantity of non-nitrogenized material. Coincident with 
muscle contraction, the blood-vessels become widely dilated, leading to 
a large increase in the blood-supply and a rapid removal of products of 
decomposition. 

Thermic Changes. — Coincident with the foregoing chemic changes and 
the transformation of energy, there is a liberation of heat and a rise in 
the temperature of the muscle. A single contraction of the gastrocne- 
mius muscle of the frog, will raise its temperature 0.00 i°C. 

Electric phenomena also manifest themselves which are similar to 
the electric phenomena presented by nerves and will be alluded to in a 
subsequent section. 

Transmission of the Contraction Wave. — Normally, when a muscle is 
stimulated by the nerve impulse, the shortening and thickening of the 
fibers begin at the end organ and travel in opposite directions to the ends 
of the muscle. This change propagates itself in a wave-like manner, 
and has been termed the contraction wave. If a stimulus be applied 
directly to the end of a long muscle, the contraction wave passes along 
its entire length to the opposite extremity, in virtue of the conductivity 
of muscular tissue. The rapidity of the propagation varies in different 
animals — in the frog, from three to four meters a second, in man, from 
ten to thirteen meters. The length of the wave varies from 200 to 400 
millimeters. 

Graphic Record of a Muscle Contraction. — The changes in the form 
of a muscle during contraction and relaxation have been carefully studied 
by recording the muscle movement by means of an attached lever, the 
end of which is allowed to rest upon a moving surface. The time rela- 
tions of all phases of the muscular movement are obtained by placing 
beneath the lever a pen attached to an electro-magnet thrown into action 
by a tuning-fork vibrating in hundredths of a second. A marking lever 
records simultaneously the moment of stimulation. 

Single Contraction. — When a single electric induction shock is applied 
to a nerve close to the muscle, the latter undergoes a quick pulsation, 

speedily returning t<> it- former condition. As shown by the muscle 
curve 1 see Fig. 1) there is between the moment of stimulation and the 



22 HUMAN PHYSIOLOGY 

beginning of the contraction a short but measurable period, known as 
the latent period, during which certain chemic changes are taking place 
preparatory to the exhibition of the muscle movement. Even when the 
electric stimulus is applied directly to the muscle, a latent period, tnough 
shorter, is observable. The duration of this period in the skeletal muscles 
of the frog has been estimated at o.oi of a second; but it has been shown 
by the employment of more accurate methods and the elimination of 
various external influences to be much less — not more than 0.0033 to 
0.0025 °f a second. 

The contraction follows the latent period. This begins slowly, rapidly 
reaches its maximum, and ceases. This has been termed the stage of 
rising or increasing energy. The time occupied in the stage of shortening 
is about 0.04 of a second, though this will depend on the strength of the 
stimulus, the load with which the muscle is weighted, and the condition 
of the muscle irritability. 




Fig. 1. — Musclb Curve Produced by a Single Induction Shock Applied to 

a Muscle. — (Landois.) 
a-f. Abscissa, a-c. Ordinate, a-b. Period of latent stimulation, b-d. Period of 
increasing energy, d-e. Period of decreasing energy, e-f. Elastic after- vibrations. 

The relaxation immediately follows the contraction. This takes place 
at first slowly, after which the muscle rapidly returns to its original 
length. This is the period of falling or decreasing energy, and occupies 
about 0.05 of a second. The whole duration of a muscle contraction 
occupies, therefore, about 0.1 of a second. 

Residual or after- vibrations are frequently seen which are due to changes 
in the elasticity of the muscle. The amplitude of the contraction depends 
upon the condition of the muscle, the load, the strength of stimulus, etc. 

Action of Successive Stimuli. — If a series of successive stimuli be applied 
to a muscle, the effect will be different according to the rapidity with 
which they follow one another. If the second stimulus be applied at the 
termination of the contraction due to the first stimulus, a second con- 
traction follows, similar in all respects to the first. A third stimulus 
produces a third contraction, and so on until the muscle becomes ex- 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE 23 

hausted. If the second stimulus be applied during either of the two 
periods of the first contraction, the effects of the two stimuli will be added 
together and the second contraction will add itself to the first. The 
maximum contraction is obtained when the second stimulus is applied 
J£o of a second after the first. 

Tetanus. — Tetanus may be defined as a more or less continuous con- 
traction of a muscle which arises when the time intervals between the 
stimuli are shorter than the time of the contraction process. Tetanus 
will be incomplete or complete according to the number of stimuli that 
reach the muscle in a second of time. When a muscle is stimulated 
directly or, better, indirectly through its related nerve by a series of 
induced currents at the rate of four or six per second, it undergoes a 
corresponding number of contractions of about equal extent. If the rate 
of stimulation is increased up to the point when the interval between each 
stimulus is less than the duration of the entire contraction process, the 
muscle does not have time to relax completely before the arrival of the 
succeeding stimulus, and hence remains in a more or less contracted state, 
during which it exhibits a series of alternate partial contractions and 
relaxations. To this condition of muscle activity the term incomplete 
tetanus or clonus is applied. 

If the stimulation be still further increased in frequency, the individual 
contractions become fused together and the curve described by the lever 
becomes a continuous line. Notwithstanding the fact that the individual 
contractions are no longer visible, it can be shown by other methods that 
the muscle is undergoing a series of slight alternate contractions and relaxa- 
tions or vibrations at least. After a varying length of time the muscle 
becomes fatigued, relaxes, and returns to its natural condition even though 
the stimulation be continued. The number of stimuli per second neces- 
sary to develop complete tetanus will depend under normal circumstances 
on the period of duration of the individual contractions. The longer this 
period, the less the number of stimuli required, and the reverse. Hence 
the number of stimuli will vary for different classes of animals and for 
different muscles in the same animal, e.g., 2 or 3 for the muscles of the 
tortoise, 10 for the muscles of the rabbit, 15 to 20 for the frog, 70 to 80 for 
birds, 330 to 340 for insects. 

Physiologic Tetanus. — A physiologic tetanus of longer or shorter 
duration may be established by an act of volition or by the action of some 
external stimulus. In the first instance the tetanus is termed volitional 
and in the second instance, reflex. 



24 HUMAN PHYSIOLOGY 

i. Volitional tetanus. As the volitional contraction is similar to that 
observed when a muscle or its related nerve is stimulated by rapidly 
repeated induced currents, it is assumed that the nerve-cells in the spinal - 
cord are discharging in a rhythmic manner a certain number of nerve 
impulses per second in consequence of the arrival of nerve impulses coming 
from the cerebral cortex, the result of volitional acts. In other words 
the volitional tetanus is the result of a discontinuous stimulation. The 
number of stimuli transmitted to a muscle during a volitional tetanus has 
been estimated by the employment of the graphic method at from 8 to 13 
per second, 10 being about the average. When a Volitional contraction 
is recorded the myogram not infrequently exhibits a series of small wave- 
like elevations which indicate that the muscle is not in a state of complete 
tetanus but is undergoing slight alternate contractions and relaxations. 
Unless the contraction process in human muscle differs from that of frogs 
it is difficult to see how 10 or even 20 stimuli per second can give rise to 
even an incomplete tetanus when the single contraction is 3^0 of a second 
in duration. 

2. Reflex. — A tetanus of muscle, physiologic in character, arises during 
the performance of many muscle movements in consequence of peripherally 
acting causes and may therefore be termed a reflex tetanus. The dura- 
tion of a tetanus thus induced, like the duration of a volitional tetanus, 
will vary with the duration of the exciting cause. Reflex tetani are pre- 
sented by the muscles of the lower jaw during mastication, by the inter- 
costal muscles during breathing, by the muscles of the limbs during 
walking, etc. In these and other instances there are reasons for believing 
that for a variable period of time the muscles are in a state of continuous 
contraction from the discharge of nerve impulses from the nerve cells in 
the spinal cord as the result of the arrival of nerve impulses coming from 
a peripheral surface. 

A non-physiologic tetanus may be excited or developed by the action of 
pharmacologic agents, e. g., strychnin, and of pathologic agents, e.g., toxins 
developed by bacteria, acting on the spinal cord mechanisms. 

Muscle Fatigue. — Prolonged or excessive muscular activity is followed 
by a diminution in the power of performing work and by an increase in the 
duration of the muscular contractions. Fatigue is accompanied by a feel- 
ing of stiffness, soreness, and lassitude, referable to the muscles themselves. 
In the early stages of muscular fatigue the contractions increased in height 
and duration, to be followed by a progressive decrease in height, but an 
increase in duration, until the muscle becomes exhausted. The cause of 
the fatigue is the production and accumulation of decomposition products, 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE 25 

such as phosphoric acid and phosphate of potassium, CO2, etc. A fatigued 
muscle is rapidly restored by the injection of arterial blood. 

Source of Muscle Energy.— According to most experimenters, it is cer- 
tain that normal muscle activity is not dependent on the metabolism of 
nitrogenous materials, inasmuch as its chief end product, urea, is not in- 
creased. The marked production of C0 2 points to the decomposition of 
some unstable compound of a carbohydrate character, rich in carbon and 
hydrogen. It has been suggested that glycogen furnishes the energy after 
it has been transformed into sugar. Muscles wanting in glycogen arc, 
nevertheless capable of contracting for some time. It has been suggested 
by Hermann that the energy of a muscular contraction may be due to the 
splitting and subsequent re-formation of a complex body belonging neither 
to the carbohydrates nor to the fats, but to the albumins. To this body 
the term inogen has been applied. This complex molecule, the product 
of the metabolic activity of the muscle cell, in undergoing decomposition 
would yield CO: sarcolactic acid, and a protein residue resembling myosin. 
With the cessation of the contraction, the muscle protoplasm recombines 
the protein residue with oxygen, carbohydrates, and fats, and again forms 
inogen. 

The phenomena of rigor mortis support such a view. At the moment 
of this contraction the muscle gives off CO2 in large amount, the muscle 
becomes acid, and myosin is formed. There is thus a close analogy 
between the two processes; in other words, a contraction is a partial 
death of the muscle. As to what becomes of the myosin formed during 
a contraction, nothing is known. It may be used in the formation 
of new inogen. 

The Production of Heat and Its Relation to Mechanical Work. — The 

transformation of energy which takes place during a muscle contraction, 
and which is dependent upon chemic changes occurring at that time, mani- 
fests itself as heat and mechanical work. While heat is being evolved con- 
tinuously during the passive condition of muscles, the amount of heat is 
largely increased during general muscle contraction. A skeletal muscle of 
a frog — e.g., the gastrocnemius — when removed from the body, shows, 
after tetanization, an increase in its temperature of from 0.14 to o.i8°C, 
and after a -ingle contraction of from 0.001 to o.oo5°C. While every 
muscular contraction is attended by an increase in heat production, the 
amount so produced will vary in accordance with certain conditions — e.g., 

ulation of blood, etc. 

ter t h« tension of a muscle, the greater, other condi- 

ing equal, is the amounl evolved. When the ends of a 



26 HUMAN PHYSIOLOGY 

muscle are fastened so that no shortening is possible during stimulation, 
the maximum of heat production is reached. In the tetanic state the 
great increase of temperature is due to the tension of antagonistic and 
strongly contracted muscles. The evolution of heat, therefore, bears a 
relation to the resistance against which the muscle is acting. 

Mechanical Work, — If a muscle contracts, loaded by a weight just suffi- 
cient to elongate it to its original length, heat is evolved, but no mechanical 
work is done, all the energy liberated manifesting itself as heat. When the 
weight which has been lifted is removed from the muscle at the height of 
contraction, external work is done. In this case the amount of heat 
liberated is less, owing to the work done, for some of the heat generated is 
transformed into mechanical motion. According to the law of the con- 
servation of energy, the amount of heat disappearing should correspond 
in heat units to the number of foot-pounds produced by muscular 
contraction. 

Work Done. — Muscles are machines capable of doing a certain amount 
of work, by which is meant the raising of a weight against gravity or the 
overcoming of some resistance. The work done is calculated by multiply- 
ing the weight by the distance through which it is raised. Thus, if a 
muscle shortens four millimeters and raises 250 grams, it does work equal 
to 1,000 milligramme ters, or one gram-meter. If a muscle contracts 
without being weighted, no work is done. Equally, when the muscle is 
over-weighted so that it is unable to contract, no work is done. The 
amount of work a muscle can do will depend upon the area of its trans- 
verse section, the length of its fibers, and the amount of the weight. The 
amount of work a laborer of 70 kilograms weight performs in eight hours 
averages 105,605 kilogram-meters, or 340.2 foot-tons. 

Muscle Sound. — Providing a muscle be kept in a state of tension during 
its contraction, the intermittent variations of its tension cause the muscle 
to emit an audible sound. If the muscle be tetanized by induction shocks, 
the pitch of the sound corresponds with the number of stimuli a second. 
A voluntary contraction is attended by a tone having a vibration fre- 
quency of about thirty-six a second, which is, however, the first overtone 
of the true muscle tone, which is caused by a contraction frequency of 
about eighteen a second. This low tone is inaudible, from the small 
number of vibrations a second. 

Rigor Mortis. — A short time after death the muscles pass into a condi- 
tion of extreme rigidity or contraction, which lasts from one to five days. 
In this state they offer great resistance to extension, their tonicity dis- 
appears, their cohesion diminishes, their irritability ceases. The time of 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE 2^ 

the appearance of this post-mortem or cadaveric rigidity varies from a 
quarter of an hour to seven hours. Its onset and duration are influenced 
by the condition of the muscular irritability at the time of death. When 
the irritability is impaired from any cause, such as disease or defective 
blood-supply, the rigidity appears promptly, but is of short duration. 
After death from acute diseases, it is apt to be delayed, but to continue 
for a longer period. 

The rigidity appears first in the muscles of the lower jaw and neck; next 
in the muscles of the abdomen and upper extremities; finally in the trunk 
and lower extremities. It disappears in practically the same order. 

Chemic changes of a marked character accompany this rigidity. The 
muscle becomes acid in reaction from the development of sarcolactic acid; 
it gives off a large quantity of carbonic acid, and is shortened and dimin- 
ished in volume. 

The immediate cause of the rigidity appears to be a coagulation of the 
myosinogcn within the sarcolemma, with the subsequent formation of 
myosin and muscle serum. In the early stages of coagulation restitution 
is possible by the circulation of arterial blood through the vessels. The 
final disappearance of this contraction is due to the action of acids dis- 
solving the myosin, and possibly to putrefactive changes. 

The Visceral Muscle. — The visceral muscle, as the name implies, is 
found in the walls of hollow viscera, where it is arranged in the form of a 
membrane or sheet. It is present in the walls of the alimentary canal, 
blood-vessels, respiratory tract, ureter, bladder, vas deferens, uterus, 
fallopian tubes, iris, etc. In some situations it is especially thick and well 
developed — e.g., uterus and pyloric end of the stomach; in others it is thin 
and slightly developed. 

The Histology of the Visceral Muscle-fiber. — When examined with 
the microscope, the muscle sheet is seen to be composed of fibers, narrow, 
elongated, and fusiform in shape. As a rule, they are extremely small, 
measuring only from 40 to 250 micromilhmeters in length and from 4 to 8 
micromilli meters in breadth. The center of each fiber presents a narrow, 
elongated nucleus. The muscle-protoplasm which makes up the body of 
the fiber appears to be enclosed by a delicate elastic membrane resembling 
in some respects the sarcolemma of the skeletal muscle. In some animals 
the visceral fiber presents a longitudinal striation suggesting the existence 
of fibrillar surrounded by sarcoplasm. The fibers are united longitudi- 
nally and transversely by a cement material. The muscle is increased in 
thickness by the superposition of successive layers. At varying intervals 
the fibers are grouped into bundles or fasciculi by septa of connective 



28 HUMAN PHYSIOLOGY 






tissue. Blood-vessels ramify in the connective tissue and furnish the 
necessary nutritive material. 

The visceral muscle receives stimuli from the spinal cord, not directly, 
however, as in the case of the skeletal muscle, but indirectly through the 
intermediation of ganglion cells, which may be located at some distance 
from the muscle or near the walls of the viscera. Non-medullated fibers 
from the ganglion pass directly into the muscle, where they frequently 
unite to form a general plexus. From this plexus fine branches take their 
origin and ultimately become physiologically associated with the muscle- 
fiber. 

Physiologic Properties. — The physiologic properties of visceral muscles 
are tonicity, elasticity, conductivity and irritability, properties which 
closely resemble the corresponding properties of the skeletal muscles. 

A contraction of the visceral muscle can be called forth by the passage 
of a single induced current and which can be graphically recorded. The 
duration of the contraction is, however, very much longer than the dura- 
tion of the skeletal muscle contraction; thus the period of shortening may 
last for five seconds and the period of relaxation for as much as thirty- 
five seconds. The muscle can also be tetanized. Moreover it will respond 
to variations in temperature, strength of stimulus, to the load in a 
manner similar to if not identical with the skeletal muscle. 

The Function of the Visceral Muscle. — In a general way it may be 
said that the visceral muscle determines and regulates the passage through 
the viscus or organ of the material contained within it. The food in the 
stomach and intestines is subjected to a churning process by the muscles, 
in consequence of which the digestive fluids are more thoroughly incor- 
porated and their characteristic action increased. At the same time the 
food is carried through the canal, the absorption of the nutritive material 
promoted, and the indigestible residue removed from the body. The 
blood is delivered in larger or smaller volumes according to the needs of 
the tissues through a relaxation or contraction of the muscle-fibers of the 
blood-vessels. The urine is forced through the ureter and from the 
bladder by the contraction of their respective muscles. The mode of 
action of the individual, muscles will be described in subsequent chapters. 

SPECIAL PHYSIOLOGY OF MUSCLES 

The individual muscles of the axial and appendicular portions of the 
body are named with reference to their shape, action, structure, etc. — 
e.g., deltoid, flexor, penniform, etc. In different localities a group of 



SPECIAL PHYSIOLOGY OF MUSCLES 29 

muscles having a common function is named in accordance with the 
kind of motion it produces or gives rise to — e.g., groups of muscles which 
alternately bend or straighten a joint, or alternately diminish or increase 
the angular distance between two bones, are known, respectively, as 
rs and extensors; such muscle groups are in association with ginglymus 
joints. Muscles which turn the bone to which they are attached around 
»wn axis without producing any great change of position are known as 
rotators, and are in association with the enarthrodial or ball-and-socket 
joints. Muscles which impart an angular movement of the extremities 
to and from the median line of the body are termed abductors and adductors. 
In addition to the actions of individual groups of muscles in causing 
ial movements in some regions, several groups of muscles are coordi- 
nated for the accomplishment of certain definite functions — e.g., muscles 
of respiration, mastication, expression. The coordination of axial and 
appendicular muscles enables the individual 
to assume certain postures, such as standing 

and sitting; to perform various acts of locomo- yy A pO) 

tion, as walking, running, swimming, etc. 



m 


F 




J, 


w 


A 




P 1 


F 


• 




1, 


A 


W 




P 1 


• 




.1 


r« 



Levers. — The function or special mode of ]> (*) 

action of individual muscles can be under- 
stood only when the bones with which they ^ ^ ^ (3) 
are connected are regarded as levers whose W PA 
fulcra or fixed points lie in the joints where the Fig. 2. — The Three Orders 
movement takes place, and when the muscles OF Levers * 
are considered as sources of power for imparting movement to the levers, 
with the object of overcoming resistance or raising weights. 

In mechanics, levers of three kinds or orders are recognized, according 
to the relative position of the fulcrum or axis of motion, the applied 
power, and the weight to be moved. (See Fig. 2.) 

In levers of the first order the fulcrum, F, lies between the weight or 
resistance, W, and the power of moving force, P. The distance P-F 
is known as the power arm, the distance W-F as the weight arm. As an 
example of this form of lever in the human body may be mentioned: 

1. The elevation of the trunk from the flexed position. The axis of 
movement, the fulcrum, lies in the hip-joint; the weight, that of the 
trunk, acting as if concentrated at its center of gravity, lies between the 
shoulders; the power, the contracting muscles attached to the tuberosity 
of the ischium. The opposite movement is equally one of the first order, 
but the relative positions of P and \Y are reversed. 

be skull in its movements backward and forward upon the atlas. 



30 HUMAN PHYSIOLOGY 

In levers of the second order the weight lies between the power and the 
fulcrum. As an illustration of this form of lever may be mentioned: 

i. The depression of the lower jaw, in which movement the fulcrum 
is the temporomaxillary articulation; the resistance, the tension of the 
elevator muscles; the power, the contraction of the depressor muscles. 

2. The raising of the body on the toes — F being the toes, W the weight 
of the body acting through the ankle, P the gastrocnemius muscle acting 
upon the heel bone. 

In levers of the third order the power is applied at a point lying between 
the fulcrum and the weight. As examples of this form of lever may be 
mentioned: 

i. The flexion of the forearm — F being the elbow-joint, P the contract- 
ing biceps and brachialis anticus muscles applied at their insertion, W 
the weight of the forearm and hand. 

2. The extension of the leg on the thigh. 

When levers are employed in mechanics, the object aimed at is the 
overcoming of a great resistance by the application of a small force acting 
through a great distance, so as to obtain a mechanical advantage. In the 
mechanism of the human body the reverse generally obtains — viz., the 
overcoming of a small resistance by the application of & great force acting 
through a small distance. As a result, there is a gain in the extent and 
rapidity of movement of the lever. The power, however, owing to its 
point of application, acts at a great mechanical disadvantage in many 
instances, especially in levers of the third order. 

Postures. — Owing to its system of joints, levers, and muscles, the 
human body can assume a series of positions of equilibrium, such as stand- 
ing, and sitting, to which the name posture has been given. In order 
that the body may remain in a state of stable equilibrium in any posture, 
it is essential that the vertical line passing through the center of gravity 
shall fall within the base of support. 

Standing is that position of equilibrium in which„a line drawn through 
the center of gravity falls within the area of both feet placed on the 
ground. This position is maintained: 

i. By firmly fixing the head on top of the vertebral column by the 
action of the muscles on the back of the neck. 

2. By making the vertebral column rigid, which is accomplished by the 
longissimus dorsi and the quadratus lumborum muscles. This having 
been accomplished, the center of gravity falls in front of the tenth dorsal 
vertebra; the vertical line passing through this point falls behind the line 
connecting both hip-joints. In consequence, the trunk is not balanced 



SPECIAL PHYSIOLOGY OF MUSCLES 3 1 

on the hip-joints, and would fall backward were it not prevented by the 
contraction of the rectus femoris muscle and ligaments. At the knees 
and ankles a similar balancing of the parts above is brought about by the 
action of various muscles. When the entire body is in the erect or 
military position, the arms by the sides, the center of gravity lies between 
the sacrum and the last lumbar vertebra, and the vertical line touches 
the ground between the feet and within the base of support. 

Sitting erect is a condition of equilibrium in which the body is balanced 
on the tubera ischii, when the trunk and head together form a rigid column. 
The vertical line passes between the tubera. 

Locomotion is the act of transferring the body, as a whole, through space, 
and is accomplished by the combined action of its own muscles. The 
acts involved consist of walking, running, jumping, etc. 

Walking is a complicated act, involving almost all the voluntary muscles 
of the body, either for purposes of progression or for balancing the head 
and trunk, and may be defined as a progression in a forward horizontal 
direction, due to the alternate action of both legs. In walking, one leg 
becomes for the time being, the active or supporting leg, carrying the 
trunk and head; the other, the passive but progressive leg, to become in 
turn the active leg when the foot touches the ground. Each leg, therefore, 
is alternately in an active and a passive state. 

Running is distinguished from walking by the fact that, at a given 
moment, both feet are off the ground and the body is raised in the air. 

While the limits of a compend do not permit of a description of the 
origin, insertion, and mode of action of the individual muscles of the 
body, it has been thought desirable to call attention to a few of the 
principal muscles whose function it is to produce special forms of move- 
ment, as well as locomotion (See Fig. 3). The erect position is largely 
maintained by the fixation of the spinal column and the balancing of 
the head upon its upper extremity; the former is accompanied by the 
erector spina muscle, named from its function and its fleshy continuations, 
situated on each side of the vertebral column. Arising from the pelvis 
and lumbar vertebrae, this muscle passes upward, and is attached by its 
continuations to all the vertebrae. Its action is to extend the vertebral 
column and to maintain the erect position. The head is balanced upon 
the top of the vertebral column by the combined action of the trapezius 
and suboccipital muscles forming the nape of the neck, and by the slerno- 
cleido-mastoid muscle. This latter muscle arises from the inner third of 
the clavicle and upper border of the sternum. It is inserted into the 
temporal bone just behind the ear. Its action is to flex the head laterally 



32 



HUMAN PHYSIOLOGY 




Fig. 3. — Superficial Muscles of the Body. 



SPECIAL PHYSIOLOGY OF MUSCLES 33 

and to rotate the face to the opposite side. When both muscles act 
simultaneously, the head and neck are Hexed upon the thorax. 

The temporal and masseter muscles, situated at the side of the head, 
arise respectively from the temporal fossa and the zygomatic arch, and 
are inserted into the ramus of the lower jaw. Their action is to close the 
mouth and to assist in mastication. The occipito-frontalis, the orbicularis 
palpebrarum, and orbicularis oris muscles are largely concerned in wrink- 
ling the forehead, closing the eyes and mouth, and in giving various 
expressions to the face. 

The deltoid is a thick, triangular muscle covering the shoulder-joint, 
-ing from the outer third of the clavicle, the acromial process, and the 
spine of the scapula, its fibers converge to be inserted into the humerus 
just above its middle point. Its action is to elevate the arm through a 
right angle. Owing to its point of insertion it acts as a lever of the third 
order, but, notwithstanding the advantageous points of insertion, it 
acts at a considerable disadvantage, owing to the obliquity of its direction. 

The biceps muscle, situated on the anterior aspect of the arm, arises 
from the upper border of the glenoid fossa and the coracoid process, and 
is inserted into the radius just beyond the elbow-joint. Its action is to 
flex and supinate the forearm and to place it in the most favorable position 
for striking a blow. When the forearm is fixed, it assists in flexing the 
arm. as in climbing. 

The triceps muscle, situated on the back of the arm, arises from the 
scapula and the posterior surface of the humerus, and is inserted in the 
olecranon process of the ulna. In its action it directly antagonizes the 
biceps, namely, extending the forearm. In so doing it acts as a lever of 
the first order. The short distance between the muscular insertion and 
the fulcrum causes it to act at a great mechanical disadvantage, but there 
is a corresponding gain in both speed and range of movement. The 
muscles of the forearm are very numerous. Their action is to impart 
to the forearm and hand a variety of movements, such as pronation, 
supination, flexion, extension, rotation, etc. 

The pectoral is major and pectoral is minor muscles form the fleshy masses 
of the breast. Arising from the inner half of the clavicle, the side of the 
sternum, and the outer surfaces of the third, fourth, and fifth ribs an- 
teriorly, the muscle-fibers converge to be inserted into the humerus and 
coracoid process. Their combined action is to adduct, flex and rotate 
the arm inward and to draw the scapula downward and forward, movements 
necessary to the folding of the arms across the chest. 

The rectus abdominis and the obliquus cxtcrnus assist in forming the 
abdominal walls. 

3 



34 HUMAN PHYSIOLOGY 

The glutei muscles are three in number, are arranged in layers, and 
form the fleshy masses known as the buttocks. They arise from the side 
of the pelvis and are attached to the femur in the neighborhood of the 
great trochanter. Their action is to extend the hips, to raise the body 
from the stooping position, and to assist in walking by firmly holding the 
pelvis on the thigh while the opposite leg is advanced in the forward 
direction. 

The rectus femortSj with its associates, the vastus internus and vastus 
externus and the crureus, forms the fleshy mass on the anterior surface of 
the thigh. The former arises from the anterior part of the ilium, the 
latter from the femur. Their common tendon, which is united to the 
patella, is continued as the ligamentum patellae, which is attached to the 
upper part of the tibia. The action of this muscular group is to extend 
the leg, to flex the thigh, and to raise the entire weight of the body, as 
in changing from the sitting to the erect position. 

The biceps femoris muscle, situated on the outer and posterior aspect 
of the thigh, arises from the tuber ischii, and is inserted into the head of 
the fibula. 

The semimembranosus and the semitendinosus muscles, situated on the 
inner and posterior aspect of the thigh, are inserted into the head of the 
tibia. Their combined action is to extend the hips and to flex the knee. 
Acting from below, they assist in raising the body from the stooping 
position. 

The gastrocnemius muscle forms the enlargement known as the calf of 
the leg. It arises by two heads from the condyles of the femur. Its 
tendon, the tendo Achillis, is inserted into the posterior surface of the 
heel bone. Its action is to extend the foot and to raise the weight of the 
body in walking and running. On the front of the leg are numerous 
muscles — e.g., tibialis anticus, peroneus longus, etc., the action of which 
is to flex the foot and to antagonize the gastrocnemius. 

PHYSIOLOGY OF NERVE TISSUE 

The nerve tissue, which unites and coordinates the various organs 
and tissues of the body and brings the individual into relationship with 
the external world, is arranged anatomically into two systems, termed the 
encephalo or cerebrospinal and the sympathetic. 

The encephalo-spinal or cerebro-spinal system consists of: 

i. The brain and spinal cord, contained within the cavities of the 
cranium and the spinal column respectively, and 



PHYSIOLOGY OF NERVE TISSUE 35 

2. The cranial and spinal nerves. 
The sympathetic system consists of: 

i. A double chain of ganglia situated on each side of the spinal column 
and extending from the base of the skull to the tip of the coccyx. 

2. Various collections of ganglia situated in the head, face, thorax, 
abdomen, and pelvis. All these ganglia are united by an elaborate 
system of intercommunicating nerves, many of which are connected 
with the cerebro-spinal system. 

HISTOLOGY OF NERVE TISSUE 

The Neuron. — The nerve tissue has been resolved by the investigations 
of modern histologists into a single morphologic unit, to which the term 
neuron has been applied. The entire nervous system has been shown 
to be but an aggregate of an infinite number of neurons, each of which 
is histologically distinct and independent. Though having a common 
origin, as shown by embryologic investigations, they have acquired a 
variety of forms in different parts of the nervous system in the course 
of development. The old conception that the nervous system consists 
of two distinct histologic elements, nerve-cells and nerve-fibers, which 
differed not only in their mode of origin, but also in their properties, their 
relation to each other, and their functions, has been entirely disproved. 

The neuron, or neurologic unit, is histologically a nerve-cell, the surface 
of which presents a greater or less number of processes in varying degrees 
of differentiation. As represented in figure 7, the neuron may be said 
to consist of: (1) The nerve-cell, neurocyte, or corpus; (2) the axon, or 
nerve process; (3) the end tufts, or terminal branches. Though these 
three main histologic features are everywhere recognizable, they exhibit 
a variety of secondary features in different situations in accordance with 
peculiarities of function. Though the nerve-cell and the nerve-fiber are 
but part of the same neuron, it is convenient at present to describe them 
separately. 

The Nerve-cell. — The nerve-cell, or body of the neuron, presents a 
variety of shapes and sizes in different portions of the nervous system. 
Originally ovoid in shape, it has acquired, in course of development, 
peculiarities of form which are described as pyramidal, stellate, peat 
shaped, spindle-shaped, etc. The size of the cell varies considerably, 
the smallest having a diameter of not more than J^ooo of an inch, the 
largest not more than J^oo of an inch. Each cell consists of granular, 



36 



HUMAN PHYSIOLOGY 



striated protoplasm, containing a distinct vesicular nucleus and a well* 
denned nucleolus. A cell membrane has not been observed. From the 
surface of the adult cell portions of the protoplasm are projected in various 
directions, which portions, rapidly dividing and subdividing, form a series 
of branches, termed dendrites or dendrons. In some situations the ulti- 
mate branches of the dendrites present short lateral processes, known as 
lateral buds, or gemmules, which impart to the branches a feathery appear- 
ance. This characteristic is common to the cells of the cortex, of the 





Fig. 4. — A. Efferent Neuron; B, Afferent Neuron. Found in both Spinal 
and Cranial Nerves. 



cerebrum, and of the cerebellum. The ultimate branches of the dendrites, 
though forming an intricate felt work, never anastomose with one another, 
nor unite with dendrites of adjoining cells. According to the number of 
axons, nerve-cells are classified as monaxonic, diaxonic, polyaxonic. 
Most of the cells of the nervous system of the higher vertebrates are 
monaxonic. In the ganglia of the posterior or dorsal roots of the spinal 
and cranial nerves, however, they are diaxonic. In this situation the 
axons, emerging from opposite poles of the cell, either remain separate 
and pursue opposite directions, or unite to form a common stem, which 



PHYSIOLOGY OF NERVE TISSUE 37 

subsequently divides into two branches, which then pursue opposite direc- 
tions. (See Fig. 4.) The nerve-cell maintains its own nutrition, and 
presides over that of the dendrites and the axon as well. If the latter 
be separated in any part of its course from the cell, it speedily degenerates 
and dies. 

The axon, or nerve process, arises from a cone-shaped projection from the 
surface of the cell, and is the first outgrowth from its protoplasm. At a 
short distance from its origin it becomes markedly differentiated from the 
dendrites which subsequently develop. It is characterized by a sharp, 
regular outline, a uniform diameter, and a hyaline appearance. In 
structure, the axon appears to consist of fine fibrillar embedded in a clear, 
protoplasmic substance. Shafer advocates the view that the fibrillar are 
exceedingly fine tubes filled with fluid. The axon varies in length from a 
few millimeters to 100 cm. In the former instance the axon, at a short 
distance from its origin, divides into a number of branches, which form an 
intricate feltwork in the neighborhood of the cell. In the latter instance 
the axon continues for an indefinite distance as an individual structure. 
In its course, however, especially in the central nervous system, it gives 
off a number of collateral branches, which possess all its histologic features. 
The long axons seem to bring the body of the cell into direct relation with 
peripheral organs, or with more or less remote portions of the nervous 
system, thus constituting association or commissural fibers. 

The more or less elongated axon becomes invested, as a rule, at a short 
distance from the cell with nucleated oblong cells, which subsequently be- 
come modified and constitute a medullary or myelin sheath. This is in- 
vested by a thin, cellular membrane — the neurilemma. These three struc- 
tures thus constitute what is known as a medullated nerve-fiber. In the 
central nervous system the outer sheath is frequently absent. In the 
sympathetic system the myelin is frequently absent, though the axon is 
inclosed by the neurilemma, thus constituting a non-medullated nerve- 
fiber. 

The end tufts or terminal organs are formed by the splitting of the axon 
into a number of filaments, which remain independent of one another and 
are free from the medullary investment. The histologic peculiarities of 
the terminal organs vary in different situations, and in many instances are 
quite complex and characteristic. In peripheral organs, as muscles, 
glands, blood-vessels, skin, mucous membrane, the tufts are in direct 
organic connection with their cellular elements. In the central nerw 
tem the tufts are in more or less intimale relation with the dendrites <>f 
adjacent neurons. 

The neurons in their totality constitute the neuron or nerve tissue. 



38 



HUMAN PHYSIOLOGY 



From the fact that they are arranged both serially and collaterally into 
a regular and connected whole, they collectively constitute the system 
known as the neuron or nerve system. The neurons moreover are grouped 
into more or less complexly organized masses termed organs which in 
accordance with their actions may be divided for convenience into central 
and peripheral organs. 

The Central Organs of the Nerve System. — The central organs con- 
sist of the encephalon and spinal cord, contained within the cavities of the 
head and spinal column respectively. They consist of neurons arranged 
in a very complex manner. In a subsequent chapter the anatomic 
arrangement of their constituent parts will be detailed. 

The Peripheral Organs of the Nerve System. — These consist of the 
cranial and spinal nerves and the sympathetic ganglia. Each nerve 
consists of a variable number of neurons united into firm bundles by con- 




Fig. s. — Transverse Section of a Nerve (Median). 
ep. Epineurium. pe. Perineurium, ed. Endoneurium. — (Landois and Stirling.) 

nective tissue which supports blood-vessels and lymphatics. The bundles 
are technically known as nerve-trunks or nerves. 

The nerve-trunks connect the brain and cord with all the remaining 
structures of the body. Each nerve is invested by a thick layer of lamel- 
lated connective tissue, known as the epineurium. A transverse section 
of a nerve shows (see Fig. 5), that it is made up of a number of small 
bundles of fibers each of which possesses a separate investment of con- 
nective tissue — the perineurium. Within this membrane the nerve-fibers 
are supported by a fine stroma — the endoneurium. After pursuing a 
longer or shorter course, the nerve trunk gives off branches, which inter- 
lace very freely with neighboring branches, forming plexuses, the fibers 
of which are distributed to associated organs and regions of the body. 



PHYSIOLOGY OF NERVE TISSUE 



39 



From their origin to their termination, however, nerve-fibers retain their 
individuality, and never become blended with adjoining fibers. 

As nerves pass from their origin to their peripheral terminations, they 
give off a number of branches, each of which becomes invested with a 
lamellated sheath — an offshoot from that investing the parent trunk. 
This division of nerve bundles and sheath continues throughout all the 
branches down to the ultimate nerve-fibers, each of which is surrounded 
by a sheath of its own, consisting of a single layer of endothelial cells. 
This delicate transparent membrane, the sheath of Henle, is separated 
from the nerve-fiber by a considerable space, in which is contained lymph 
destined for the nutrition of the fiber. Near their ultimate terminations 
the nerve-fibers themselves undergo division, so that a single fiber may 
give origin to a number of branches, each of which contains a portion of 
the parent axis-cylinder and myelin. 

The neurons composing the spinal and cranial nerves are represented in 
Fig. 6, which are connected peripherally by their terminal branches with 




Fig. 6. — Diagram of a Simple Reflex Arc 
Sentient surface. 2. Afferent nerve. 3. Emissive or motor cell. 4. Efferent 
nerve. 5. Muscle. — {After Morat and Dayon.) 



muscles on the one hand and with epithelium of skin, mucous membrane, 
etc., on the other hand. In the spinal cord the terminal branches of the 
afferent neuron come into histologic and physiologic relation with the 
dendrites of a second neuron, the axonic process of which in many instances 
ascends the cord to different levels or even as far as the brain, where its 
terminal branches come into relation with the dendrites of still another 
neuron, the axonic process of which is in turn connected with neurons in 
the cortex of either the cerebrum or cerebellum. The surfaces of the body 
are thus brought into relation with the cerebral and cerebellar neurons. 
The neurons arranged in this serial manner constitute the afferent side of 
the nerve system. 



40 HUMAN PHYSIOLOGY 

In a similar way the efferent neurons of the spinal and cranial nerves are 
brought into relation with the cortex of the cerebrum. Large pyramidal- 
shaped neurocytes situated in specialized regions of the cortex of the cere- 
brum send their axonic processes down through the brain and cord. As 
they approach their destination the terminal branches become related 
histologically and physiologically with the dendrites of the neurons com- 
posing the cranial and spinal nerves. The cortex of the cerebrum is thus 
brought into relation with the general musculature of the body. The 
The neurons arranged in this serial manner constitute the efferent side of 
the nerve system. 

Sympathetic Ganglia. — A sympathetic ganglion consists essentially of 
a connective-tissue capsule with an interior framework. The meshes of 
this framework contain nerve-cells possessing dendrites and branching 
axons. The majority of the axons are devoid of myelin and are therefore 
known as non-myelineated nerve-fibers. Owing to the absence of the 
myelin they present a rather pale or grayish appearance. In all instances, 
with the exception of the ganglion cells of the heart, the axons are dis- 
tributed to non-striated muscle tissue and to the epithelium of glands. 

The nerve-cells of the ganglia are also in histologic connection with the 
terminal branches of certain fine medullated nerve-fibers which leave the 
spinal cord by way of the ventral roots of the spinal nerves. These nerve- 
fibers are designated pre- ganglionic fibers, while those emerging from the 
cells are designated post-ganglionic fibers. 

THE RELATION OF THE PERIPHERAL ORGANS OF THE 
NERVE SYSTEM TO THE CENTRAL ORGANS 

Spinal Nerves. — The nerves in connection with the spinal cord are 
thirty-one in number on each side. If traced toward the spinal column, 
it will be found that the nerve-trunk passes through an intervertebral 
foramen. Near the outer limits of the foramina each nerve-trunk divides 
into two branches, generally termed roots, one of which, curving slightly 
forward and upward, enters the spinal cord on its anterior or ventral sur- 
face, while the other, curving backward and upward, enters the spinal 
cord on its posterior or dorsal surface. The former is termed the anterior 
or ventral root; the latter, the posterior or dorsal root. Each dorsal root 
presents near its union with the ventral root a small ovoid grayish enlarge- 
ment known as a ganglion. Both roots previous to entering the cord 
subdivide into from four to six fasciculi. 

A microscopic examination of a cross-section of the spinal cord shows 
that the fibers of the ventral roots can be traced directly into the body of 



PHYSIOLOGY OF NERVE TISSUE 4 1 

the nerve-cells in the ventral horns of the gray matter. The fibers of the 
dorsal roots are not so easily traced, for they diverge in several directions 
shortly after entering the cord. In their course they give off collateral 
branches which, in common with the main fiber, and in tufts which become 
►dated with nerve-cells in both the ventral and dorsal horns of the 
gray matter. 

Cranial Nerves. — The nerves in connection with the base of the brain 
are known as cranial nerves; some of these nerves present a similar gan- 
glionic enlargement, and therefore may be regarded as dorsal nerves, while 
others may be regarded as ventral nerves. Their relations within the 
medulla oblongata are similar to those within the spinal cord. 

Development and Nutrition of Nerves. — The efferent nerve-fibers, 

which constitute some of the cranial nerves and all the ventral roots of 
the spinal nerves, have their origin in cells located in the gray matter 
beneath the aqueduct of Sylvius, beneath the floor of the fourth ventricle 
and in the anterior horns of the gray matter of the spinal cord. These 
cells are the modified descendants of independent, oval, pear-shaped 
cells — the neuroblasts — which migrate from the medullary tube. As 
they approach the surface of the cord their axons are directed toward 
the ventral surface, which eventually they pierce. Emerging from the 
cord, the axons continue to grow, and become invested with the myelin 
sheath and neurilemma, thus constituting the ventral roots. 

The afferent nerve-fibers, which constitute some of the cranial nerves 
and all the dorsal roots of the spinal nerves, develop outside of the 
central nervous system and only subsequently become connected with 
it. At the time of the closure of the medullary tube a band or ridge 
of epithelial tissue develops near the dorsal surface, which, becoming 
segmented, moves outward and forms the rudimentary spinal ganglia. 
The cells in this situation develop two axons, one from each end of the 
cell, which pass in opposite directions, one toward the spinal cord, the 
other toward the periphery. In the adult condition the two axons shift 
their position, unite, and form a T-shaped process, after which a division 
into two branches again takes place. In the ganglia of all the sensori- 
cranial and sensorispinal nerves the cells have this histologic peculiarity. 

Efferent and Afferent Nerves. — Xerves are channels of communica- 
tion between the brain and spinal cord, on the one hand, and the skeletal 
muscles, glands, blood-vessels, visceral muscles, skin, mucous membrane, 
| etc., on the other. Some of the nerve-fibers serve for the transmission <»f 
nerve energy from the brain and spinal cord to certain peripheral organs, 



42 HUMAN PHYSIOLOGY 

and so accelerate or retard, augment or inhibit their activities; others 
serve for the transmission of nerve energy from certain peripheral organs 
to the brain and spinal cord which gives rise to sensation or other modes 
of nerve activity. The former are termed efferent or centrifugal, the latter 
afferent or centripetal nerves. Experimentally it has been determined 
that the anterior or ventral roots contain all the efferent fibers, the posterior 
or dorsal roots all the afferent fibers. 

The efferent nerves may be classified, in accordance with their dis- 
tribution and the characteristic forms of activity to which they give rise, 
into several groups, as follows: 

i. Skeletal-muscle or motor nerves, those which convey nerve energy or 
nerve impulses directly to skeletal-muscles and excite them to activity. 

2. Gland or secretor nerves, those which convey nerve impulses to glands 
by way of ganglia and influence in one direction or another the degree of 
their activity. Those which cause the formation and discharge of the 
secretion peculiar to the gland are known as secreto-motor, while those 
which decrease or inhibit the secretion are known as secreto-inhibitor 
nerves. 

3. Vascular or vaso-motor nerves, those which convey nerve impulses 
to the muscle-fibers of the blood-vessels and change in one direction or 
the other the degree of their natural contraction. Those which increase 
the contraction are known as vaso-constrictors or vaso-augmentors; those 
which decrease the contraction are known as vaso-dilatators or vaso- 
inhibitors. The nerves which pass to that specialized part of the vascular 
apparatus, the heart, transmit nerve impulses which on the one hand 
accelerate its rate or augment its force, and on the other hand inhibit 
or retard its rate and diminish its force. For this reason they are termed 
cardiac nerves, one set of which is known as cardio-accelerator and cardio- 
augmentor, the other as cardio-inhibitor nerves. 

4. Visceral or viscero-motor nerves, those which transmit nerve impulses 
to the muscle walls of the viscera and change in one direction or another 
the degree of their contraction. Those which increase or augment the 
contraction are known as viscero-augmentor, while those which decrease 
or inhibit the contraction are known as viscero-inhibitor nerves. 

5. Hair bulb or pilo-motor nerves, those which transmit nerve impulses 
to the muscle-fibers which cause an erection of the hairs. 

Of the foregoing nerves the skeletal-muscle or motor nerves alone pass 
directly to the muscle. The gland, the vascular and the visceral nerves, 



PHYSIOLOGY OF NERVE TISSUE 43 

all terminate at a variable distance from the peripheral organ around a 
local sympathetic ganglion, which in turn is connected with the peripheral 
brgan. The former are termed pre-ganglionic, the latter post-ganglionic 
fibers. 

The afferent nerves may also be classified, in accordance with their 
distribution and the character of the sensations or other modes of nerve 
activity to which they give rise, into several groups, as follows: 

A. Tcgumcntary nerves, comprising those distributed to skin, mucous 
membranes and sense organs and which transmit nerve impulses from 
the periphery to the nerve centers. They may be divided into reflex 
and sensorifacient nerves. 

i. Reflex nerves, those which transmit nerve impulses to the spinal 
cord and medulla oblongata, where they give rise to different modes 
of nerve activity. They may be divided into: 

a. Reflex excitator nerves, which transmit nerve impulses which 
cause an excitation of nerve centers and, in consequence, in- 
creased activity of peripheral organs, e.g., skeletal muscles, 
glands, blood-vessels and viscera. 

b. Reflex inhibitor nerves, which transmit nerve impulses which 
cause an inhibition of nerve centers and, in consequence, de- 
creased activity of the peripheral organs. It is quite probable 
that one and the same nerve may subserve both sensation and 
reflex action, owing to the collateral branches which are given 
off from the afferent roots as they ascend the posterior column 
of the cord. 

2. Sensorifacient nerves, those which transmit nerve impulses to the 
brain where they give rise to conscious sensations. They may be 
sub-divided into: 

a. Nerves of special sense — e.g., olfactory, optic, auditory, gus- 
tatory, tactile, thermal, pain, pressure — which give rise to cor- 
respondingly named sensations. 

b. Nerves of general sense — e.g. } the visceral afferent nerves — those 
which give rise normally to vague and scarcely perceptible 
sensations, such as the general sensations of well-being or dis- 
comfort, hunger, thirst, fatigue, sex, want of air, etc. 

B. Muscle nerves, comprising those distributed to muscles and tendons 
and which transmit nerve impulses from muscles and tendons to 
the brain, where they give rise to the so-called muscle sensations, 

, the direction and the duration of a movement, the resistance 
offered and the posture of the body or of its individual parts. 



44 HUMAN PHYSIOLOGY 

The foregoing classification of the efferent and afferent nerve-fibers 
has been established partly by experiment and partly by histologic 
investigations, e.g. 

Stimulation of the ventral, efferent root fibers produces : 

i. Tetanic contraction of skeletal muscles. 

2. Discharge of secretions from glands. 

3. Increase in the degree of the contraction, the tonus, of the muscle 
walls of the peripheral arteries. 

4. Variations in the degree of the contraction, the tonus, of the muscle 
walls of certain viscera either in the way of augmentation or inhibition. 1 

Division of the ventral root fibers is followed by: 

1. Relaxation of skeletal muscles and loss of movement. 

2. Cessation in the discharge of secretions from glands. 

3. Temporary dilatation and loss of the tonus of blood-vessels. 

4. Temporary impairment of the normal activities of the visceral 
muscles from loss of central nerve control; the degree of impairment 
depending on the nature of the viscus involved. 

Peripheral stimulation of the dorsal afferent root fibers produces : 

1. Reflex excitation of spinal centers, in consequence of which there is 
an increased activity of skeletal muscles, glands, blood-vessels, and vis- 
ceral walls. 

2. Reflex inhibition of spinal nerve-centers, in consequence of which 
there may be a decrease in the activities of skeletal muscles, glands, blood- 
vessels, and viscera. 

3. Sensations of touch, temperature, pressure, and pain. 

4. Sensations of the duration and direction of muscle movements, of 
the resistance offered and of the position of the body or of its individual 
parts (muscle sensations). 

Division of the dorsal root fibers is followed by: 

1. Loss of the power of exciting or inhibiting reflexly the activities of 
spinal nerve-centers and in consequence a loss of the power of exciting or 
inhibiting the activities of peripheral organs. 

2. Loss of sensation in all parts to which they are distributed. 

1 These last three phenomena are especially associated with the ventral roots of 
the second thoracic to the third or fourth lumbar nerves inclusive. 



PHYSIOLOGY OF NERVE TISSUE 45 

The ventral roots are, therefore, efferent in function, transmitting nerve 
impulses from the spinal cord to the peripheral organs which excite them 
to activity. 

The dorsal roots are afferent in function, transmitting nerve impulses 
from the general periphery to {a) the spinal cord where they excite its con- 
tained nerve-centers to activity or to a more or less complete cessation of 
activity ^inhibition), and (b) to the cerebrum where they excite its centers 
to activity with the development of sensations. 

The peripheral terminations of the efferent nerves are, therefore, to be 
found in close histologic relation with skeletal-muscle and visceral-muscle 
fibers and with gland epithelium. The peculiar termination in each situa- 
tion has been termed an "end organ. " The afferent nerves are likewise in 
close histologic relation with the skin, mucous membrance and the sense 
organs. The afferent end organs are in some instances extremely complex, 
such as those found in the eye (retina), the internal ear, the nose and 
tongue. 

The end organs of the afferent nerves are specialized, highly irritable 
structures placed between the nerve-fibers and the surface of the body. 
They are especially adapted for the reception of those external forces 
technically known as stimuli, and for the liberation of energy capable of 
exciting the nerve-fiber to activity. 

Nerve Degeneration. — If any one of the cranial or spinal nerves be 
divided in any portion of its course, the part in connection with the peri- 
phery in a short time exhibits certain structural changes, to which the term 
degeneration is applied. The portion in connection with the brain or cord 
retains its normal condition. The degenerative process begins simulta- 
neously throughout the entire course of the nerve, and consists in a disin- 
tegration and reduction of the medulla and axis-cylinder into nuclei, drops 
of myelin, and fat, which in time disappear through absorption leaving 
the neurilemma intact. Coincident with these structural changes there 
is a progressive alteration and diminution in the excitability of the nerve. 
Inasmuch as the central portion of the nerve, which retains its connection 
with the nerve-cell, remains histologically normal, it has been assumed that 
the nerve-cells exert over the entire course of the nerve-fibers a nutritive 
or a trophic influence. This idea has been greatly strengthened since the 
discovery that the axis-cylinder, or the axon, has its origin in and is a 
direct outgrowth of the cell. \Yhen separated from the parent cell, t lie- 
fiber appears to be incapable of itself of maintaining its nutrition. 

The relation of the nerve-cells to the nerve-fibers, in reference to their 
nutrition, is demonstrated by the results which follow section of the ventral 
and dorsal roots of the spinal nerves. If the anterior root alone be divided, 



46 HUMAN PHYSIOLOGY 

the degenerative process is confined to the peripheral portion, the central 
portion remaining normal. If the posterior root be divided on the peri- 
pheral side of the ganglion, degeneration takes place only in the peripheral 
portion of the nerve. If the root be divided between the ganglion and the 
cord, degeneration takes place only in the central portion of the root. 
From these facts it is evident that the trophic centers for the ventral and 
dorsal roots lie in the spinal cord and spinal nerve ganglia, respectively, or, 
in other words, in the cells of which they are an integral part. The struc- 
tural changes which nerves undergo after separation from their centers 
are degenerative in character, and the process is usually spoken of, after 
its discoverer, as the Wallerian degeneration. 

When the degeneration of the efferent nerves is completed, the struc- 
tures to which they are distributed, especially the muscles, undergo an 
atrophic or fatty degeneration, with a change or loss of their irritability. 
This is, apparently, not to be attributed merely to inactivity, but rather to 
a loss of nerve influences, inasmuch as inactivity merely leads to atrophy 
and not to degeneration*. 

Reactions of Degeneration. — In consequence of the degeneration and 
changes in irritability which occur in nerves when separated from their 
centers and in muscles when separated from their related nerves, either 
experimentally or as the result of disease, the response of these structures 
to the induced and the make-and-break of the constant currents differs 
from that observed in the physiologic condition. The facts observed 
under the application of these two forms of electricity are of the greatest 
importance in the diagnosis and therapeutics of the precedent lesions. 
The principal difference of behavior is observed in the muscles, which 
exhibit a diminished or abolished excitability to the induced current, while 
at the same time manifesting an increased excitability to the constant 
current; so much so is this the case that a closing contraction is just as 
likely to occur at the positive as at the negative pole. This peculiarity 
of the muscle response is termed the reaction of degeneration. The syn- 
chronous diminished excitability of the nerves is the same for either cur- 
rent. The term "partial reaction of degeneration" is used when there is 
a normal reaction of the nerves, with the degenerative reaction of the 
muscles. This condition is observed in progressive muscular atrophy. 

PHYSIOLOGIC PROPERTIES OF NERVES 

Nerve Irritability or Excitability and Conductivity. — These terms are 
employed to express that condition of a nerve which enables it to develop 
and to conduct nerve impulses from the center to the periphery, from the 



PHYSIOLOGY OF NERVE TISSUE 47 

periphery to the center, in response to the action of stimuli. A nerve is 
said to be excitable or irritable as long as it possesses these capabilities or 
properties. For the manifestation of these properties the nerve must 
retain a state of physical and chemic integrity; it must undergo no change 
in structure or chemic composition. The irritability of an efferent nerve 
is demonstrated by the contraction of a muscle, by the secretion of a gland, 
or by a change in the caliber of a blood-vessel, whenever a corresponding 
nerve is stimulated. The irritability of an afferent nerve is demonstrated 
by the production of a sensation or a reflex action whenever it is stimulated. 
The irritability of nerves continues for a certain period of time after separa- 
tion from the nerve centers and even after the death of the animal, vary- 
ing in different classes of animals. In the warm-blooded animals, in 
which the nutritive changes take place with great rapidity, the irritability 
soon disappears — a result due to disintegrative changes in the nerve, 
caused by the withdrawal of the blood-supply. In cold-blooded animals, 
on the contrary, in which the nutritive changes take place relatively slowly, 
the irritability lasts, under favorable conditions, for a considerable time. 
Other tissues besides nerves possess irritability, that is, the property of 
responding to the action of stimuli — e.g., glands and muscles, which re- 
spond by the production of a secretion or a contraction. 

Independence of Tissue Irritability. — The irritability of nerves is 
distinct and independent of the irritability of muscles and glands, as shown 
by the fact that it persists in each a variable length of time after their 
histologic connections have been impaired or destroyed by the introduction 
of various chemic agents into the circulation. Curara, for example, in- 
duces a state of complete paralysis by modifying or depressing the conduc- 
tivity of the end organs of the nerves just where they come in contact with 
the muscles without impairing the irritability of either nerve trunks or mus- 
cles. Atropin induces complete suspension of glandular activity by im- 
pairing the terminal organs of the secretor nerves just where they come 
into relation with the gland cells, without destroying the irritability of 
either gland or nerve. 

Stimuli of Nerves. — Nerves do not possess the power of spontaneously 
generating and propagating nerve impulses; they can be aroused to activity 
only by the action of an extraneural stimulus. In the living condition 
the stimuli capable of throwing the nerve into an active condition act 
for the most part on either the central or peripheral end of the nerve. In 
the case of motor nerves the stimulus to the excitation, originating in some 
molecular disturbance in the nerve-cells, acts upon the nerve-fibers in 
connection with them. In the case of sensor or afferent nerves the stimuli 



48 HUMAN PHYSIOLOGY 

act upon the peculiar end organs with which the sensor nerves are in con- 
nection, which in turn excite the nerve-fibers. Experimentally, it can 
be demonstrated that nerves can be excited by a sufficiently powerful 
stimulus applied in any part of their extent. 

Nerves respond to stimulation according to their habitual function; 
thus, stimulation of a sensor nerve, if sufficiently strong, results in the sen- 
sation of pain; of the optic nerve, in the sensation of light; of a motor nerve, 
in contraction of the muscle to which it is distributed; of a secretor nerve, 
in the activity of the related gland, etc. It is, therefore, evident that 
peculiarity of nerve function depends neither upon any special construc- 
tion or activity of the nerve itself, nor upon the nature of the stimulus, 
but entirely upon the peculiarities of its central and peripheral end organs. 

Nerve stimuli may be divided into — 

i. General stimuli, comprising those agents which are capable of exciting 
a nerve in any part of its course. 

2. Special stimuli, comprising those agents which act upon nerves only 
through the intermediation of the end organs. 

General stimuli: j 

i. Mechanical: as from a blow, pressure, tension, puncture, etc. 

2. Thermal; heating a nerve at first increases and then decreases its 
excitability. 

3. Chemic: sensor nerves respond somewhat less promptly than motor 
nerves to this form of irritation. 

4. Electric: either the constant or interrupted current. 

5. The normal physiologic stimulus: 

(a) Centrifugal or efferent, if proceeding from the center toward the 
periphery. 

(b) Centripetal or afferent, if in the reverse direction. 

Special stimuli: 

1. Light or ethereal vibrations acting upon the end organs of the optic 
nerve in the retina. 

2. Sound or atmospheric undulations acting upon the end organs of 
the auditory nerve. 

3. Heat or vibrations of the air upon the end organs in the skin. 

4. Chemic agencies acting upon the end organs of the olfactory and 
gustatory nerves. 



PHYSIOLOGY OF XF.RYE TISSUE 49 

Nature of the Nerve Impulse. — As to the nature of the nerve impulse 
rated by any of the foregoing stimuli either general or special, but 
little is known. It has been supposed to partake of the nature of a mole- 
cular disturbance, a combination of physical and chemical processes 
nded by the liberation of energy, which propagates itself from mole- 
cule to molecule. Judging from the deflections of the galvanometer needle 
it is probable that when the nerve impulse makes its appearance at any 
en point it is at first feeble but soon reaches a maximum development 
r which it speedily declines and disappears. It may, therefore, be 
graphically represented as a wave-like movement with a definite length 
and time duration. Under strictly physiological conditions the nerve 
impulse passes in one direction only; in efferent nerves from the center to 
the periphery, in afferent nerves from the periphery to the center. Experi- 
mentally, however, it can be demonstrated that when a nerve impulse is 
aroused in the course of a nerve by an adequate stimulus it travels equally 
well in both directions from the point of stimulation. ^Yhen once started 
the impulse is confined to the single fiber and does not diffuse itself to 
libers adjacent to it in the same nerve trunk. 

Rapidity of Transmission of Nerve Force. — The passage of a nervous 
impulse, either from the brain to the periphery or in the reverse direction, 
requires an appreciable period of time. The velocity with which the 
impulse travels in human sensor nerves has been estimated at about 190 
feet a second, and for motor nerves at from 100 to 200 feet a second. The 
rate of movement is, however, somewhat modified by temperature, cold 
lessening and heat increasing the rapidity; it is also modified by electric 
conditions, by the action of drugs, the strength of the stimulus, etc. The 
rate of transmission through the spinal cord is considerably slower than in 
nerves, the average velocity for voluntary motor impulses being only 33 
feet a second, for sensitive impressions 40 feet, and for tactile impressions 
140 feet a second. 

Electric Currents in Muscles and Nerves. — If a muscle or nerve be 
divided and non-polarizable electrodes be placed upon the natural longi- 
tudinal surface at the equator, and upon the transverse section, electric 
currents are observed with the aid of a delicate galvanometer. The direc- 
tion of the current is always from the positive equatorial surface to the 
negative transverse surface. The strength of the current increases or 
diminishes according as the positive electrode is moved toward or from 
the equator. When the ele< re placed on the two transverse ends 

nerve, an axial current will be observed the direction of which is op- 
posite to that of the normal impulse of the nerve. 
4 



SO HUMAN PHYSIOLOGY 

The electromotive force of the strongest nerve-current has been estimated 
to be equal to the 0.026 of a Daniell battery; the force of the current of the 
frog muscle, about 0.05 to 0.08 of a Daniell. 

Negative Variation of Currents in Muscles and Nerves. — If a muscle 
or nerve be thrown into a condition of tetanus, it will be observed that the 
currents undergo a diminution of negative variation, a change which passes 
along the nerve in the form of a wave and with a velocity equal to the rate 
of transmission of the nerve impulse. The wave-length of a single nega- 
tive variation has been estimated to be eighteen millimeters, the period 
of its duration being from 0.0005 to 0.0008 of a second. 

It is asserted by Hermann that perfectly fresh, uninjured muscles and 
nerves are devoid of currents, and that the currents observed are the result 
of molecular death at the point of section, this point becoming negative to 
the equatorial point. He applies the term "action currents" to the 
currents obtained when a muscle is thrown into a state of activity. 

Electrotonus. — The passage of a direct galvanic current through a portion 
of a nerve excites in the parts beyond the electrodes a condition of electric 
tension, or electrotonus, during which the excitability of the nerve is de- 
creased near the anode or positive pole, and increased near the cathode or 
negative pole; the increase of excitability in the catelectrotonic area — that 
nearest the muscle — being manifested by a more marked contraction of 
the muscle than the normal when the nerve is irritated in this region. The 
passage of an inverse galvanic current excites the same condition of elec- 
trotonus; the diminution of excitability near the anode, the anelectrotonic 
— that now nearest the muscle — being manifested by a less marked con- 
traction than the normal when the nerve is stimulated in this region. 
Similar conditions exist within the electrodes. Between the electrodes is 
a neutral point, where the catelectrotonic area merges into the anelectro- 
tonic area. If the current be a strong one, the neutral point approaches 
the cathode; if weak, it approaches the anode. 

When a nerve impulse passes along a nerve, the only appreciable effect 
is a change in its electric condition, there being no change in its tempera- 
ture, chemic composition, or physical condition. The natural nerve- 
currents, which are always present in a living nerve as a result of its 
nutritive activity, in great part disappear during the passage of an impulse, 
undergoing a negative variation. 

Law of Contraction. — If a feeble galvanic current be applied to a recent 
and excitable nerve, contraction is produced in the muscles only upon the 
making of the circuit with both the direct and inverse currents. 

If the current be moderate in intensity, the contraction is produced in 



PHYSIOLOGY OF NERVE TISSUE 5 1 

the muscle, both upon the making and breaking of the circuit, with both the 
direct and inverse currents. 

If the current be intense, contraction is produced only when the circuit 
is made with the direct current, and only when it is broken with the inverse 
current. 

Reflex Action. — Inasmuch as many of the muscle movements of the 
body, as well as the formation and discharge of secretions from glands, 
variations in the caliber of blood-vessels, inhibition and acceleration in 
the activity of various organs, are the result of stimulations of the terminal 
organs of afferent nerves, they are termed, for convenience, reflex actions, 
and, as they take place independently of the brain or of volitional impulses, 
they are also termed involuntary actions. As many of the processes to 
be described in succeeding chapters are of this character, requiring for 
their performance the cooperation of several organs and tissues associated 
through the intermediation of the nervous system, it seems advisable to 
consider briefly, in this connection, the parts involved in a reflex action, 
as well as their mode of action. As shown in figure 10, the necessary 
structures are as follows: 

i. A receptive surface, skin, mucous membrane, sense organ, etc. 

2. An afferent nerve. 

3. An emissive cell, from which arises 

4. An efferent nerve, distributed to a responsive organ, as, 

5. Muscle, gland, blood-vessel, etc. 

Such a combination of structures constitutes a reflex mechanism o 
arc the nerve portion of which is composed of but two neurons — an 
afferent and an efferent. An arc of this simplicity would of necessity 
subserve but a simple movement. The majority of reflex activities, how- 
ever, are extremely complex, and involve the cooperation and coordination 
of a number of structures frequently situated at distances more or less 
remote from one another. This implies that a number of neurons are 
associated in function. The afferent neurons are brought into relation 
with the dendrites of the efferent neurons by the end tufts of the collateral 

• branches, which may extend for some distance up and down the cord 

I before passing into the various segments. 

For the excitation of a reflex action it is essential that the stimulus ap- 
plied to the sentient surface be of an intensity sufficient to develop in the 
terminals of the afferent nerve a series of nerve impulses, which, traveling 
inward, will be distributed to and received by the dendrites of the emissive 
motor cell. With the r< f these impulses there is apparently a 

disturbance of the equilibrium of its molecules, a liberation of energy, and 



52 HUMAN PHYSIOLOGY 

in consequence, a transmission outward of impulses through the efferent 
nerve to muscle, gland, or blood-vessel, separately or collectively, with the 
production of muscular contraction, glandular secretion, vascular dilata- 
tion or contraction, etc. The reflex actions take place, for the most part, 
through the spinal cord and medulla oblongata, which, in virtue of their 
contained centers, coordinate the various organs and tissues concerned in 
the performance of the organic functions. The movements of mastication; 
the secretion of saliva; the muscular, glandular, and vascular phenomena 
of gastric and intestinal digestion; the vascular and respiratory move- 
ments; the mechanism of micturition, etc., are illustrations of reflex 
activity. 

FOODS AND DIETETICS 

During the functional activity of every organ and tissue of the body the 
living material of which it is composed — the protoplasm — undergoes more 
or less disintegration. Through a series of descending chemic stages it is 
reduced to a number of simpler compounds, which are of no further value 
to the body, and which are in consequence eliminated by the various 
eliminating or excretory organs — the lungs, kidneys, skin, liver. Among 
these compounds the more important are carbon dioxid, urea, and uric 
acid. Many other compounds, inorganic as well as organic, are also 
eliminated in the water discharged from the body, in which they are held 
in solution. Coincident with this disintegration of the tissue there is an 
evolution or disengagement of energy, particularly in the form of heat. 

In order that the tissues may regain their normal composition and thus 
be enabled to continue in the performance of their functions, they must be 
supplied with the same nutritive -materials of which their protoplasm 
originally consisted — viz., water, inorganic salts, proteins, sugar, fat. 
These materials are furnished by the blood during its passage through 
the capillary blood-vessels. The blood is a reservoir of nutritive material 
in a condition to be absorbed, organized, and transformed into new living 
tissue. 

Inasmuch as the loss of material from the body daily, which is very 
great, is compensated for under other forms by the blood, it is evident that 
this fluid would rapidly diminish in volume were it not restored by the 
introduction of new and corresponding materials. As soon as the blood 
volume falls to a certain point, the sensations of hunger and thirst arise, 
which in a short time lead to the necessity of taking food. 

In addition to the direct appropriation of food by the tissues it is highly 
probable that an indefinite amount undergoes oxidation and disintegra- 



FOODS AND DIETETICS . $3 

tion without ever becoming an integral part of the tissues, and thus directly 
contributes to the production of heat. 

Quantities of Food Materials Required Each Day. — The quantities of 
the different nutritive materials that are required each day for the growth 
and repair of the tissues and for the evolution of heat have been variously 
estimated by different observers. The following table shows the average 
diet scale of Vierordt and the amounts of the waste products to which it 
would give rise: 

Comparison of the Income and Outcome 

Income Outcome 

Protein 120 grams. Water 3,1 14 .00 

Fat 90 grams. ] Urea 33 .80 

Starch 330 grams. Urinary solids. . . . > Salts 26 .00 

Inorganic salts 32 grams. J Extractives 6.00 

■ r 2,818 grams. Feces 44 .00 

Oxygen 744 grams. Carbon dioxid 910.00 



4.134 grams. 4,134.00 

It will be observed that in the results of the foregoing experiment, 
the amount of water under outcome, exceeds the amount under income, 
by 296 grams. This water results from the union of a portion of the 
oxygen absorbed with the surplus hydrogen of the fats. If the diet 
consisted merely of protein and starch the two volumes of water would 
practically balance each other. 

Many other attempts have been made to construct a suitable diet for 
a man weighing 70 kilos while doing light or moderate work. The 
following are accepted estimates: 



Ranke, Voit, Moleschott, 
grams grams grams 


At water, 
grams 


Protein. .... too 118 130 

Fat. . 100 56 84 
Starch. 250 500 550 


1 -'5 
125 
400 



The Energy of the Animal Body. — The food consumed daily not only 
repairs the loss of material from the body, but also furnishes the energy to 
replace that which is expended daily in the shape of heat and motion. 
All the energy of the body can be traced to the chemic changes going on 
in the tissues, and more particularly to those changes involved in the 
oxidation of the foo< 



54 HUMAN PHYSIOLOGY 

The amount of heat yielded by any given food principle can be deter- 
mined by burning it to carbon dioxid and water, arid ascertaining the 
extent to which it will, when so liberated, raise the temperature of a given 
volume of water. This amount of heat may be expressed in Calories. 
A Calorie is the amount of heat required to raise the temperature of one 
kilogram of water one degree Centigrade. 

The following estimates give, approximately, the number of Calories 
produced when the food is reduced within the body to urea, carbon dioxid, 
and water: 

i gram of protein yields 4.124 kilogram Calories. 

1 gram of fat yields 9-353 kilogram Calories. 

1 gram of starch yields 4. 116 kilogram Calories. 

The total number of kilogram Calories yielded by any given diet scale 
can be readily determined by multiplying the preceding factors by the 
quantities of material consumed. The diet scale of Ranke, for example, 
yields the following amount: 

100 grams of protein yield 412.4 Calories. 

100 grams of fat yield 935*3 Calories. 

240 grams of starch yield 987.8 Calories. 

Total 2,335.5 Calories. 

It has also been determined experimentally that one gram of protein, 
one gram of fat, and one gram of starch, when completely oxidized, will 
yield energy sufficient to perform, 1,850, 3,841, and 1,567 kilogrammeters 
of work, respectively. A kilogrammeter of work is one kilogram raised 
one meter high. 

The total energy of the Ranke diet scale can be easily calculated — e.g., 



100 grams of protein yield 185,000 kilogrammeters. 

100 grams of fat yield 384,100 kilogrammeters. 

240 grams of starch yield 397.680 kilogrammeters. 

Total 966,780 kilogrammeters. 



It will be thus seen that the food consumed daily yields 2,335 kilogram 
Calories, which can be translated into its mechanical equivalent, 966,780 
kilogrammeters of work. 



FOODS AND DIETETICS 55 

CLASSIFICATION OF FOOD PRINCIPLES 
i. Proteins. 

Principle Where found 

Myosin Flesh of animals. 

Yitellin, albumin Yolk of egg, white of egg. 

Fibrin, globulin Blood contained in meat. 

Caseinogen Milk, cheese. 

Gliadin and glutinin Grain of wheat and other cereals. 

Vegetable albumin Soft, growing vegetables. 

Legumin Peas, beans, lentils, etc. 

2. Fats. 

Animal fats and oils ] Found in the adipose tissue of ani- 

Stearin, olein \ mals, seeds, grains, nuts, fruits 

Palmitin, fat acids J and other vegetable tissues. 

3. Carbohydrates. 

Saccharose, or cane-sugar Sugar-cane. 

Dextrose, or glucose \ ^ . 

_ , * . > Fruits. 

Levulose, or fruit-sugar J 

Lactose, or milk-sugar Milk. 

Maltose Malt, malt foods. 

Starch Cereals, tuberous roots, and legu- 
minous plants. 
Glycogen Liver, muscles. 

4. Inorganic Principles. — Water; sodium and potassium chlorids; 
sodium calcium, magnesium, and potassium phosphates; calcium carbon- 
ate; and iron. 

5. Vegetable Acids. — Malic, citric, tartaric, and other acids, found 
principally in fruits. 

6. Accessory Foods. — Tea, coffee, alcohol, cocoa, etc. 

DISPOSITION OF FOOD 

The Proteins. — The protein principles of the food while in the ali- 
mentary canal undergo a series of disintegrative changes by virtue of which 
they are reduced in part to simple nitrogen-holding bodies, monoamino- 
and diamino-acids and ammonia, and in part to their immediate anteced- 
ents peptids and polypeptids, after which they are absorbed from the 
intestinal contents. Recently evidence has been adduced which makes it 
probable" that the amino-acids undergo no change in the act of absorption 
but enter the blood as such and are carried direct to the tissues. On 
reaching any given tissue the cells absorb and synthesize, perhaps under 



56 HUMAN PHYSIOLOGY 

the influence of an enzyme, such amino-acids as they may need for their 
growth and repair. The surplus amino-acids, i.e., those not utilized in 
the synthesis of tissue protein, may be synthesized to plasma-albumin, 
or stored unchanged or be deaminized, i.e., separated perhaps by the 
action of an enzyme, into the amino-group, NH 2 , and some carbonaceous 
radical. The amino-group is then combined with hydrogen, and sub- 
sequently with carbon dioxid, to form ammonium carbonate which 
is then transformed into urea, a transformation that takes place to some 
extent in the muscles (Folin) ; the carbonaceous remainder may be trans- 
formed into fat or sugar, which is subsequently oxidized thus contributing 
to the production of heat. In the process of tissue metabolism the protein 
molecule undergoes disintegration and gives rise to amino-acids, the 
different elements of which may undergo changes similar to those just 
stated. The ammonia absorbed from the intestine is changed to ammon- 
ium carbonate carried direct to the liver and transformed into urea. 

The Fats. — The fat principles while in the alimentary canal also undergo 
a series of changes whereby they are reduced by enzymic action to soap 
and glycerin, under which forms they are absorbed. During the act of 
absorption the soap and glycerin are synthesized to human fat. The fine 
particles thus formed in the intestinal wall are carried by the lymph vessels 
to the thoracic duct, and thence into the blood stream, from which they 
rapidly disappear. Though it is possible that a portion of the fat enters 
directly into the formation of the living material in general, it is generally 
believed that it is at once oxidized and reduced to carbon dioxid and water 
with the liberation of energy. The natural supposition that a portion of 
the synthesized fat is directly stored up in the cells of the areolar connect- 
ive tissue, thus giving rise to adipose tissue, has been a subject of much 
controversy, though modern experimentation renders this very probable. 
The body-fat, under physiologic conditions, is mainly, however, a product 
of the transformation of carbohydrates. 

The Carbohydrates. — Carbohydrate principles are reduced during di- 
gestion to simple forms of sugar, chiefly dextrose and levulose. Under 
these forms they are absorbed into the blood. These compounds are then 
carried to the liver and to the muscles where they are dehydrated and 
stored under the form of starch, termed animal starch or glycogen. Sub- 
sequently glycogen is transformed by hydration to sugar, after which it 
is oxidized to carbon dioxid and water. The intermediate stages through 
which sugar passes before it is reduced to carbon dioxid and water are 
only imperfectly known. Though a large part of the carbohydrate 
material is at once oxidized, it is now well established that another por- 



FOODS AND DIETETICS 57 

tion contributes to the formation of, if it is not directly converted into. fat. 
the carbohydrates form a large portion of the food, they contribute 
materially to the liberation of energy. 

The Inorganic Principles. — The inorganic principles, though apparently 
not playing as active a part in the metabolism of the body as the organic, 
are nevertheless essential to its physiologic activity. 

r is present in all the fluids and solids of the body. It promotes the 

►rption of new material from the alimentary canal; it holds the various 

ingredients of the blood, lymph, and other fluids in solution; it hastens the 

rption of waste products from the tissues, and promotes their speedy 

elimination from the body. 

Sodium chlorid is present in all parts of the body to the extent of no 
gm. The average amount eliminated daily is 15 gm. Its necessity as an 
article of diet is at once apparent. Taken as a condiment, it imparts 
sapidity to the food, excites the flow of the digestive fluids, influences 
the passage of nutritive material through animal membranes, and fur- 
nishes the chlorin for the free hydrochloric acid of the gastric juice. In 
some unknown way it favorably promotes the activity of the general 
nutritive process. 

The potassium salts are also essential to the normal activity of the 
nutritive process. When deprived of these salts, animals become weak 
and emaciated. When given in small doses, they increase the force of 
the heart-beat, raise the arterial pressure, and thus increase the action of 
the circulation of the blood. 

The calcium phosphate and carbonate are utilized in imparting solidity to 
the tissues, more especially the bones and teeth. Many articles of food 
contain these salts in quantities sufficient to restore the amount lost 
daily. 

The vegetable acids increase the secretions of the alimentary canal, and 
are apt, in large amounts, to produce flatulence and diarrhea. After 
entering into combination with bases to form salts, they stimulate the 
action of the kidneys and promote a greater elimination of all the urinary 
constituents. In some unknown way they influence nutrition; when 
deprived of these acids, the individual becomes scorbutic. 

The accessory foods, coffee and tea, when taken in moderation, overcome 
the sense of fatigue and mental unrest consequent on excessive physical 
and mental exertion. Coffee increases the action of the intestinal glands 
and acts a? a laxativ< Vfter absorption, its active principle, caffein, . 

ulates the action of the heart, raise* the arterial pressure, and ex< ites 
the action of the brain. Tea acts as an astringent, owing to the tannic 



58 HUMAN PHYSIOLOGY 

acid it contains. One effect of the tannic acid is to coagulate the digestive 
ferments and to interfere with the activity of the digestive process. 

Alcohol when taken in small quantities stimulates the digestive glands 
to increased activity and thus promotes digestive power. Its absorption 
into the blood is followed by increased action of the heart, dilatation of 
the cutaneous blood-vessels, a sensation of warmth, and an excitation of 
the brain. In large quantities it acts as a paralyzant, depressing more 
especially the vaso-constrictor nerve-centers and certain areas of the brain, 
as shown by an impairment in the power of sustained attention, clearness 
of judgment, and muscle coordination. 

Alcohol is undoubtedly oxidized in the body, as only about 2 per cent, 
can be obtained from the urine and expired air. It thus contributes to the 
store of the body-energy. Whether for this reason it can be regarded 
as a food — that is, whether it can be substituted in part at least for fat or 
carbohydrate material without impairing the protein metabolism — is at 
present a subject of experimentation and discussion. According to some 
investigators, alcohol does not retard protein metabolism, for when it is 
introduced into the body in amounts equivalent to the carbohydrates with- 
drawn from the food there is at once a rise in the amount of nitrogen 
excreted. Hence it cannot be regarded as a food. According to other 
investigators, alcohol retards or protects protein metabolism just as 
effectually as an equivalent amount of starch or sugar. Many more 
experiments are required to decide this question. When taken habitually 
in large quantities, alcohol deranges the activities of the digestive organs, 
lowers the body temperature, impairs muscle power, lessens the resistance 
to depressing external conditions, diminishes the capacity for sustained 
mental work, and leads to the development of structural changes in the 
connective tissues of the brain, spinal cord, and other organs. In infec- 
tious diseases and in cases of depression of the vital powers it is most use- 
ful as a restorative agent. 

Inanition or Starvation. — If these nutritive principles be not supplied 
in sufficient quantity, or if they are withheld entirely, a condition of physio- 
logic decay is established, to which the term inanition or starvation is 
applied. The phenomena which characterize this pathologic process are 
as follows — viz., hunger, intense thirst, gastric and intestinal uneasiness 
and pain, muscle weakness and emaciation, a diminution in the quantity 
of carbon dioxid exhaled, a lessening in the amount of urine and its con- 
stituents excreted, a diminution in the volume of the blood, an exhalation 
of a fetid odor from the body, vertigo, stupor, delirium, and at times con- 
vulsions, a fall of bodily temperature, and, finally, death from exhaustion. 



FOODS AND DIETETICS 59 

During starvation the loss of different tissues, before death occurs, 
averages ^o, or 40 per cent., of their weight. 

Those tissues which lose more than 40 per cent, are: Fat, 93.3; blood, 75; 
spleen, 71.4; pancreas, 64.1; liver, 52; heart, 44.8; intestines, 42.4; muscle, 
42.3. Those which lose less than 40 per cent, are: The muscular coat of 
the stomach, 39.7; pharynx and esophagus, 34.2; skin, 33.3; kidneys, 
31.9; respiratory apparatus, 22.2; bones, 16.7; eyes, 10; nervous system, 
1.9. 

The fat entirely disappears, with the exception of a small quantity 
which remains in the posterior portion of the orbits and around the kidneys. 
The blood diminishes in volume and loses its nutritive properties. The 
muscles undergo a marked diminution in volume and become soft and 
flabby. The nervous syste?n is last to suffer, not more than two per cent., 
disappearing before death occurs. 

The appearances presented by the body after death from starvation are 
those of anemia and great emaciation; almost total absence of fat; blood- 
lessness; a diminution in the volume of the organs; an empty condition of 
the stomach and bowels, the coats of which are thin and transparent. 
There is a marked disposition of the body to undergo decomposition, giving 
rise to a very fetid odor. 

The duration of life after a complete deprivation of food varies from eight 
to thirteen days, though life can be maintained much longer if a quantity 
of water be obtained. The water is more essential under these circum- 
stances than the solid matters, which can be supplied by the organism 

itself. 



COMPOSITION OF FOODS 

The food principles essential to the maintenance of the nutrition of 
.he body are contained in varying proportions in compound substances 
:ermed foods; e.g., meat, milk, wheat, potatoes, etc. Their nutritive value 
depends partly on the amounts of their contained food principles and 
tartly on their digestibility. The dietary of civilized man embraces foods 
lerived from both the animal and vegetable worlds. 

The following tables show the percentage composition of the edible 
x>rtions of foods as well as the amount of heat liberated per pound when 
oxidized in the body, according to Atwater and Bryant. 

Composition of Animal Foods. — The following table shows the average 
percentage composition of various kinds of meats, cow's milk, and eggs: 



6o 



HUMAN PHYSIOLOGY 



Kind of food 
materials 



Water 



Unavail- 
able 
nutrients 



Pro- 
teins 



Fat 



Car- 
bohy- 
drates 



Ash 



Fuel value 

per lb., 
453-6 grams 



Beef: 

Loin, lean 

^ Loin, fat 

Round, lean.. . . 

Round, fat 

Veal: 

Cutlets (round) 

Liver 

Mutton: 

Leg 

Loin 

Pork: 

Loin chops 

Ham 

Fowl: 

Turkey... 

Mackerel 

Halibut 

Milk 

Eggs, boiled 



Per 

cent. 

67.0 
54-7 
70.0 
60.4 

70.7 
73-0 

62.8 
50.2 

52.0 
53-9 



63 .7 
55-5 
73-4 
75-4 
87.0 
73-2 



Per 

cent. 



1 .2 
1.9 



1.6 



1-3 

0.9 



1-7 
2.4 



Per 

cent. 

19. 1 
17.0 
20.7 
18.9 

19.7 
9-7 

17.9 
IS- 5 

16. 1 
14.8 
18.7 
20.5 

18. 1 

18.0 

3.2 

12.8 



Per 
cent. 

12 .1 

26.2 

7-5 

18.5 

7-3 
5-0 

17. 1 
3i.4 

28.6 

27-5 

15-5 

21.8 

6.7 

4-9 

3-8 

11. 4 



Per 
cent. 



50 



Per 

cent. 

1 .0 
0.9 

1 .1 
1 .0 



0.6 



Calories 



900 

i,470 

735 

1,175 

710 
410 

1,095 
1,660 

1,555 
1,480 
1,040 
853 
650 
570 
310 
755 



Composition of Cereal Foods. — The average composition of the prin- 
cipal cereals is shown in the following table: 



Kind of food 
material 



Water 



Unavail- 
able 
nutrients 



Pro- 
teins 



Fat 



Car- 
bohy- 
drates 



Ash 



Fuel 
value per 
lb., 453.6 

grams 



Entire wheat flour . 

Rye flour 

Rice 

Barley, pearled. . . . 
Buckwheat flour. . . 

Corn meal 

Oat meal 

Whole wheat bread. 

White bread 

Graham crackers. . . , 



Per 
cent. 
11. 4 

9 

3 
5 
6 
5 



Per 

cent. 
4-5 



Per 

cent. 

10.7 

53 

6.5 

6 

5 

7 

13 
7 
7 

7 



Per 

cent. 
1.7 
0.8 
0.3 

1 .0 

1 .1 
1.7 
6.6 
0.8 

1 .2 
8.5 



Per 
cent. 
70.9 
76.9 
76.9 
76.10 
75-9 
735 
65.2 
49-1 
52.3 
72.5 



Per 

cent. 

0.8 

0.5 

0.3 

0.8 

0.7 



Calories 

1,645 
1,610 
1,610 
1,630 
1,600 
1,625 
1,795 
1,125 
1,195 
1,900 



DIGESTION 



6l 



Composition of Vegetable Foods. — The average composition of some 
of the principal vegetables is shown in the following table: 



Kind of food 
material 



Unavail- 
Water able 

nutrients 



Pro- 
teins 



Per 
cent. 
10.4 
68.5 
12 .6 
95-3 



Beans, lima, dried.. . 
Beans, lima, green. . 
Beans, white, dried.. 
Beans, string, cooked 

Peas, dried 9.5 

Peas, green, cooked 1 . . . 73 . 8 
Potatoes, boiled, 

cooked 1 75 .5 

Potatoes, sweet 51.0 

Beets, cooked 1 88.6 

Cabbage 91. 5 

Tomatoes 94-3 

Turnips 89.6 

Egg-plant 92 .9 

Spinach, fresh 92 .3 

Asparagus, cooked ... 91 .6 



Per 

cent. 
6.7 
2 .7 
75 
0.5 
7-6 
2.5 

1-7 
3-0 
1 .2 
0.7 
0.4 
0.8 
0.6 
1 .0 



Per 
cent. 
12.8 

53 
15-8 

0.6 
17.3 

51 



0.7 
1 .0 
0.9 
1.6 
1-7 



Fat 



Car- 
bohy- 
drates 



Ash 



Fuel 
value per 
lb., 453.6 

grams 



Per 

cent. 



Per 

cent. 



1.4 I 65.6 
c .6 21 .6 
1.6 j 59-9 

1.0 j 1.9 
0.9 62.5 

3.1 14-4 



20.0 
40.3 
7.2 
5-5 
3-8 
7-8 
4-9 
3-2 
2 . 1 



Per 
cent. 
3-1 
1.3 

2.6 
0.7 
2 .2 
1 . 1 

0.8 



Calories 

1,565 
525 

i,530 
90 

1,508 
490 

415 
885 
170 
140 
100 
175 
120 
100 
195 



iWith butter, etc., added. 






DIGESTION 



Digestion is a physical and chemic process by which the food introduced 
into the alimentary canal is liquefied and its nutritive principles trans- 
formed by the digestive fluids into new substances capable of being ab- 
sorbed into the blood. 

The digestive apparatus consists of the alimentary canal and its appen- 
dages — viz., teeth, lips and tongue; the salivary, gastric and intestinal 
glands; the liver and pancreas. 

Digestion may be divided into several stages; prehension, mouth 
digestion (mastication and insalivation), deglutition, gastric and intestinal 
digestion, and defecation. 

Prehension, the act of conveying food into the mouth, is accomplished 

•by the hand-, lips, and teeth. 



62 HUMAN PHYSIOLOGY 

Mouth Digestion. — Mastication is the mechanical division of the food, 
and is accomplished by the teeth and the movements of the lower jaw under 
the influence of muscular contraction. When thoroughly divided, the 
food presents a larger surface for the solvent action of the digestive fluids, 
thus enabling them to exert their respective action more effectively and 
in a shorter period of time. 

The teeth are thirty-two in number, sixteen in each jaw, and divided 
into four incisors or cutting teeth, two canines, four bicuspids, and six 
molars or grinding teeth; each tooth consists of a crown covered by enamel, 
a neck, and a root surrounded by the crusta petrosa and embedded in the 
alveolar process; a section through a tooth shows that its substance is 
made of dentine, in the center of which is the pulp cavity containing blood- 
vessels and nerves. 

The lower jaw is capable of making a downward and an upward, a lateral 
and an anteroposterior movement, dependent upon the construction of the 
temporomaxillary articulation. 

The jaw is depressed by the contraction of the digastric, geniohyoid, 
mylohyoid, and platysma myoides muscles; elevated by the temporal, 
masseter, and internal pterygoid muscles; moved laterally by the alternate 
contraction of the external pterygoid muscles; moved anteriorly by the 
pterygoid, and posteriorly by the united actions of the geniohyoid, mylohyoid, 
and posterior fibers of the temporal muscles. 

The food is kept between the teeth by the intrinsic and extrinsic muscles 
of the tongue from within, and the orbicularis oris and buccinator muscles 
from without. 

The movements of mastication, though originating in an effort of the 
will and under its control, are, for the most part, of an automatic or reflex 
character, taking place through the medulla oblongata and induced by the 
presence of food within the mouth. The nerves and nerve-centers in- 
volved in this mechanism are shown in the following table: 

Nerve Mechanism of Mastication 



Afferent nerves 


Efferent nerves 


I. Lingual branches of the tri- 


i. Small root of the trigeminal 


geminal nerve. 


nerve. 


2. Glossopharyngeal. 


2. Hypoglossal. 




3. Facial. i 



The impressions made upon the terminal filaments of the afferent nerves 
are transmitted to the medulla; motor impulses are here generated which 
are transmitted through the efferent nerves to the muscles involved in the 



DIGESTION 63 

movements of the lower jaw. The medulla not only generates motor 
impulses, but coordinates them in such a manner that the movements of 
mastication may be directed toward the accomplishment of a definite 
purpose. 

Insalivation. — Insalivation is the incorporation of the food with the 
saliva secreted by the parotid, submaxillary, and sublingual glands; the 
parotid saliva, thin and watery, is poured into the mouth through Steno's 
duct; the submaxillary and sublingual salivas, thick and viscid, are poured 
into the mouth through Wharton's and Bartholin's ducts respectively. 

In their minute structure the salivary glands resemble one another. 
They belong to the racemose variety, and consist of small sacs or vesicles, 
which are the terminal expansions of the smallest salivary ducts. Each 
vesicle or acinus consists of a basement membrane surrounded by blood- 
vessels and lined with epithelial cells. In the parotid gland the lining 
cells are granular and nucleated; in the submaxillary and sublingual glands 
the cells are large, clear, and contain a quantity of mucigen. During and 
after secretion very remarkable changes take place in the cells lining the 
acini, which are in some way connected with the essential constituents of 
the salivary fluids. 

In the living serous glands — e.g., parotid — during rest, the secretory 
cells lining the acini of the gland are seen to be filled with fine granules, 
which are often so abundant as to obscure the nucleus and enlarge the cells 
until the lumen of the acinus is almost obliterated. When the gland 
begins to secrete the saliva, the granules disappear from the outer 
boundary of the cells, which then become clear and distinct. At the end of 
the secretory activity the cells have been freed of granules and have 
become smaller and more distinct in outline. It would seem that the 
granular matter is formed in the cells during the period of rest and dis- 
charged into the ducts during the activity of the gland. 

In the mucous glands — e.g., submaxillary and sublingual — the changes 
that occur in the cells are somewhat different. During the inter- 
vals of digestion the cells lining the gland are large, clear, and 
highly refractive, and contain a large quantity of mucigin. After secre- 
tion has taken place the cells exhibit a marked change. The mucigin 
cells have disappeared, and in their place are cells which are small, dark, 
and composed of protoplasm. It would appear that the cells, during rest, 
elaborate the mucigin, which is discharged into the tubules during secretory 
activity, to become part of the secretion. 

Mouth Saliva. — The saliva found in the mouth is an opalescent, slightly 
id, alkaline fluid, having a specific gravity of 1.005. Microscopic 



6 4 



HUMAN PHYSIOLOGY 



examination reveals the presence of salivary corpuscles and epithelial 
cells. Chemically it is composed of water protein materials and inorganic 
salts. The amount secreted daily has been estimated at about 2 lb. 

Physiologic Action. — Experiments have shown that saliva has a two- 
fold action, viz., physical and chemical. 

1. Physically saliva moistens and softens the food, unites its particles 
into a consistent mass and thus facilitates swallowing. 

2. Chemically it converts boiled starch into sugar. It has a feeble if 
any action on raw starch by reason of the structure of the starch granule. 
Each granule consists of two portions, an envelope of cellulose and a 
contained material granuloses the true starch material. When subjected 
to the action of boiling water the granule swells and bursts forming a 
more or less viscid fluid, starch paste. If saliva be now added to this paste 
and kept at a temperature of about ioo°F. for a few minutes, the paste 
becomes clear and liquid. The first stage in the digestion of starch is 
now complete with the formation of soluble starch. If the action of saliva 
be continued, substances intermediate between starch and sugar are 
formed to which the name dextrin has been given, e.g.. 



Starch = Soluble starch 



r_ ^ . A . / Achroodextrin 
'Erythroderma 

I Maltose 



The erythrodextrin is so called because it gives rise to a red color with 
iodin. Achroodextrin is so called because it yields no color with iodin. 

The sugar formed by the action of saliva is the compound sugar maltose 
the formula for which is C12H22O11. This chemical action of saliva 
depends on the presence of an unorganized ferment or enzyme known as 
ptyalin. 

Nerve Mechanism of Insalivation. — The afferent and efferent nerves 
that constitute the nerve mechanism of insalivation are shown in the 
following tabulation: 

Nerve center 
Medulla 
oblongata. 



Afferent nerves 

1. Lingual branches of the 
trigeminal nerve. 

2. Taste fibers in the glosso- 
pharyngeal. 

3. Taste fibers in the chorda 
tympani. 



Efferent nerves 
Auriculotemporal branch 
of the trigeminal nerve, 
for parotid gland. 
Chorda tympani, for sub- 
maxillary and sublingual 
glands. 

Sympathetic for all the 
glands. 



DIGESTION 65 

The nerve centers exciting, through efferent nerves the secretion of saliva 
are located in the medulla oblongata and may be aroused to action (1) by 
nerve impulses descending from the brain in consequence of psychic states 
induced by the sight and odor of food and (2) by nerve impulses reflected 
through afferent nerves from the mouth developed by the taste of food. 
The afferent nerves thus stimulated in the second instance are those stated 
in the foregoing tabulation. 

That the efferent nerves in the same tabulation are active in the produc- 
tion of the secretion is shown by the following facts: 

Stimulation of the auriculotemporal branch increases the flow of saliva 
from the parotid gland; division arrests it. 

Stimulation of the chorda tympani is followed by a dilatation of the 
blood-vessels of the submaxillary and sublingual glands, an increased flow 
of blood and an abundant discharge of saliva; division of the nerve arrests 
the secretion. 

Stimulation of the cervical sympathetic is followed by a contraction of 
the blood-vessels, a diminished flow of blood, and a diminution of the 
secretion, which now becomes thick and viscid; division of the sympathetic 
is not, however, followed by complete dilatation of the vessels. There is 
evidence of the existence of a local vasomotor mechanism, which is 
inhibited by the chorda tympani. 

Deglutition. — Deglutition is the act of transferring food from the mouth 
into the stomach, and may be divided into three stages: 

1. The passage of the bolus from the mouth into the pharynx. 

2. From the pharynx into the esophagus. 

3. From the esophagus into the stomach. 

In the first stage, which is entirely voluntary, the mouth is closed and 
respiration momentarily suspended; the tongue, placed against the roof 
of the mouth, arches upward and backward, and forces the bolus into 
the fauces. 

The second and third stages, or the passage of the food through the 
pharynx and esophagus into the stomach, have been attributed until quite 
recently entirely to peristaltic movements of their musculature. 

Recent experiments have demonstrated that deglutition consists of two 
phases: (1) a rapid rise of pressure in the pharynx, as a result of which 
liquid or semi-liquid foods are suddenly shot down to the lower end of the 
esophagus; (2) a peristaltic contraction of the musculature of the canal, 
which, acting as a supplementary force, carries onward any particles of 
food in the canal and forces the bolus through the closed sphincter cardice 
at the end of the esophagus. 
I s 



66 HUMAN PHYSIOLOGY 

The immediate cause of the sudden rise of pressure was shown to be 
the contraction of the mylohyoid muscles. When the nerves going to 
these muscles were divided in a dog, deglutition was practically abolished. 
These muscles are probably assisted in their action by the contraction 
of the hyoglossus muscles as well as the tongue itself. 

The time required for a mouthful of liquid food to pass to the lower end 
of the esophagus is approximately about o.i second. If the cardiac orifice 
is normally closed, a period of about 6 or 7 seconds may elapse before the 
oncoming peristaltic wave reaches the lower end of the esophagus and 
forces the fluid into the stomach. If, however, a series of deglutitory acts 
follow one another in quick succession there is an inhibition of the cardiac 
sphincter and the peristaltic wave, until after the last swallow. The time 
required for the food to pass into the stomach varies in different animals 
and in different human beings. 

The Closure of the Posterior Nares and Larynx. — Because of the rapid 
rise of pressure in the pharynx and esophagus during the act of swallowing 
the posterior nares and the opening of the larynx must be closed to prevent 
the food from entering them. 

The posterior nares are closed against the entrance of the food by a 
septum formed by the pendulous veil of the palate and the posterior half 
arches. The palate is drawn upward and backward by the levator palati 
muscles, until it meets the posterior wall of the pharynx, which at this 
moment advances. At the same time it is made tense, by the action 
of the tensor palati muscles. This septum is completed by the ad- 
vance toward the middle line of the posterior half arches caused by the 
contraction of the muscles, the palato-pharyngei, which compose them. 
When these structures are impaired in their functional activity, as in 
diphtheritic paralysis and ulcerations, there is not infrequently a regurgi- 
tation of food, especially liquids, into the nose. 

The larynx is equally protected against the entrance of food during 
deglutition under normal circumstances. That this accident occasionally 
happens, giving rise to severe spasmodic coughing, and even in extreme 
cases to suffocation, is abundantly shown by the records of clinical medi 
cine. Usually it does not occur, for the following reasons: just preceding 
and during the act of deglutition there is a complete suspension of the act 
of inspiration, by which particles of food might otherwise be drawn into 
the larynx; at the same time the larynx is always drawn well up under the 
base of the tongue and its entrance closed by the downward and backward 
movement of the epiglottis. 

In addition to the downward and backward movement of the epiglottis 
and the ascent of the larynx under the base of the tongue, it is also probable 



DIGESTION 67 

that the larynx is protected from the entrance of food, in the rabbit at 
least, by the closure of the glottis itself. 

GASTRIC DIGESTION 

The Stomach. — Immediately beyond the termination of the esophagus 
the alimentary canal expands and forms a receptacle for the temporary re- 
Lention of the food. To this dilatation the term stomach has been applied. 
This organ is somewhat pyriform in outline, and occupies the upper part of 
:he abdominal cavity. It is about 25 to 35 centimeters long, 15 centi- 
neters deep, and 10 to 12 centimeters wide, and has a capacity of about 
[500 c.c. It presents two orifices, the cardiac or esophageal, and the 
)yloric; two curvatures, the lesser and the greater. 

The general body of the stomach has been divided into two portions, 
iz., a large portion to the left, known as the cardiac portion and a small 
>ortion to right known as the pyloric portion. The extreme left end of 
he stomach is somewhat enlarged and forms the fundus. 

The stomach walls are formed by three coats: 

1. The serous, a reflection of the peritoneum. 

2. The muscular, the fibers of which are arranged in a longitudinal, a 
ircular, and an oblique direction. In the pyloric portion of the stomach 
he circular fibers increase enormously in number and form thick, well- 
efined rings termed the pyloric muscles. At the pyloric orifice the 
mscle fibers form a distinct band termed the sphincter pylori. The 
rifice between the lower end of the esophagus and stomach is also closed 
y a sphincter known as the sphincter cardice. 

3. The mucous, which is somewhat larger than the muscular coat, and 
1 consequence is thrown into folds or rugae. The surface of the mucous 
Dat is covered by tall, narrow, columnar epithelium. 

Gastric Glands. — Embedded within the mucous membrane are to be 
>und enormous numbers of tubular glands, which though resembling one 
nother in general form, differ in their histologic details in various portions 
: the stomach. 

In the cardiac end or fundus, the glands consist of several long tubules 
JCning into a short, common duct, which opens by a wide mouth on the 
irface of the mucous membrane. Each gland consists primarily of a 
lsement membrane lined by epithelial cells. In the duct the epithelium 

of the columnar variety, resembling that covering the surface of the 

ucous membrane. The secretory portion of the tubule is lined by a 
yer of short, polyhedral, granular, and nucleated cells, which, as they 



68 



HUMAN PHYSIOLOGY 



border the lumen of the tubule, and thus occupy the central portion oi 
the gland, are termed central cells. At irregular intervals, between the 
central cells and the wall of the tubule, are found large oval, reticulated 
cells, which, on account of their position, are termed parietal cells. (See 
Fig. 7.) 

Each parietal cell is in relation with a system of fine canals, which open 
directly into the lumen of the gland. It is estimated that the fundus 




Fig. 7. — Diagram showing the relation of the ultimate twigs of the blood-vessels 
V and A and of the absorbent radicles to the glands of the stomach and the differen 
kinds of epithelium — viz., above cylindric cells; small, pale cells in the lumen 
outside which are the dark ovoid cells. — (Yeo.) 



contains about five million glands. In the pyloric end of the stomacl 
the glands are generally branched at their lower extremities, and th» 
common duct is long and wide. The duct is lined by columnar epi 
thelium, while the secreting part is lined by short, slightly columnar 
granular cells. The parietal cells are entirely wanting. The epitheliun 
covering the surface of the mucous membrane is tall, narrow and cylindri': 
in shape, and consists of mucus-secreting goblet cells. The outer half 
the cell contains a substance, mucinogen, which produces mucin. Th« 






DIGESTION 69 

gastric glands in both situations are surrounded by a line connective 
tissue, which supports blood-vessels, nerves, and lymphatics. 

Changes in the Cells during Secretion. — During the periods of rest 
and secretory activity the cells of the glands undergo changes in structure 
which are supposed to be connected with the production of the pepsin 
and hydrochloric acid. During rest, the protoplasm of the central cells 
becomes^nlled with granular matter; during the time of secretion this 
disappears, presumably passing into the lumen of the tubule, and as a 
result the protoplasm becomes clear and hyalin in appearance. The 
granular material is probably the mother substance, pepsinogen, which, 
inactive in itself, yields the active ferment, pepsin. The parietal cells 
during digestion increase in size, but do not become granular. It is at 
this period that they secrete the hydrochloric acid. After digestion they 
rapidly diminish in size and return to their former condition. The pyloric 
glands secrete pepsin only. 

Gastric Juice. — The gastric juice obtained from the human stomach 
free from mucus and other impurities is a clear, colorless fluid with a con- 
stant acid reaction, a slightly saline and acid taste, and a specific gravity 
varying from 1.002 to 1.005. When kept from atmospheric influences, 
it resists putrefactive change for a long period of time, undergoes no 
apparent change in composition, and loses none of its digestive power. 
It will also prevent and even arrest putrefactive change in organic matter. 
The chemic composition of the gastric juice has never been satisfactorily 
determined, owing to the fact that the secretion as obtained from fistulous 
Dpenings has not been absolutely normal. It may however be said to 
:onsist of water, organic matter, hydrochloric acid and various inorganic 
^alts. The quantitative composition of the juice varies somewhat in 
different animal^- 

The organic matter present in gastric juice is a mixture of mucin and a 
protein, products of the metabolic activity of the epithelial cells on the 
>urface of the mucous membrane and of the chief or central cells of the 
gastric glands respectively. Associated with the protein material are 
possibly three ferment or enzyme bodies, termed pepsin, rennin and 
.ipase. As is the case with other enzymes, their true chemic nature is 
:ically unknown. 

psin. — Pepsin, though present in gastric juice, is not present as such 
n the chief cells of the glands, but is derived from a zymogen, propepsin 
>r pepsinogen, when the latter is treated with hydrochloric acid. This 
intecedent compound is related to the granules observed in and produced 
•jy the cell protoplasm during the period of rest. Though pepsin is 



70 HUMAN PHYSIOLOGY 

largely produced by the central cells of the cardiac glands, it is also 
produced, though in less amount, by the cells of the pyloric glands. 
Pepsin is the chief proteolytic or proteoclastic agent of the gastric juice 
and exerts its influence most energetically in the presence of hydrochloric 
acid and at a temperature of about 4o°C. 

Rennin. — Rennin or pexin is present in the gastric juice not only of man 
and all of the mammalia, but also of birds and even fish. In its origin 
from a zymogen substance; in its relation to an acid medium and an 
optimum temperature it bears a close resemblance to pepsin. Its specific 
action is the coagulation of milk, a condition due to a transformation of 
soluble caseinogen into a solid flaky body, casein. 

Lipase. — Lipase, an enzyme found in pancreatic juice, has also been 
shown to be present in gastric juice, the specific function of which appears 
to be the digestion or hydrolysis of finely emulsified fat such as is found 
in milk. 

Hydrochloric Acid. — Hydrochloric acid is the agent which gives to the 
gastric juice its normal acidity. Though the juice frequently contains 
lactic, acetic, and even phosphoric acids, it is generally believed that 
they are the result of fermentation changes occurring in the food, the j 
result of bacterial action. The percentage of hydrochloric acid has been 
the subject of much discussion. The most recent investigations show j 
that the initial acidity of the freshly secreted human gastric juice is be- 
tween 0.32 and 0.48 per cent. HC1. This initial acidity is reduced by j 
combination with food, admixture with saliva and gastric mucus, and by 
regurgitation of alkaline duodenal contents, to 0.15 or 0.2 per cent. HC1, 
the optimum acidity for the proteolytic activity of pepsin. As observed 
clinically, following various test meals, the acidity of the gastric contents 
is seen to rise to a maximum as digestion progresses, after which it falls 
to the optimum point of about 0.2 per cent. HC1. 

Hydrochloric acid exerts its influence in a variety of ways. It is the 
main agent in the derivation of pepsin and rennin or pexin from their 
antecedent zymogen compounds, pepsinogen and pexinogen (Warren); 
it imparts activity to these ferments; it prevents and even arrests fer- 
mentative and putrefactive changes in the food by destroying micro- 
organisms; it softens connective tissue, it dissolves and acidifies the ', 
proteins, thus making possible the subsequent action of pepsin. 

The inorganic salts of the gastric juice are probably only incidental ; 
and play no part in the digestive process. 

Mechanism of Secretion. — Modern investigations have established the 
fact that the production and the discharge of gastric juice is under the 
control of a nerve center situated in the medulla oblongata. From this 



DIGESTION 71 

center nerve fibers pass by way of the vagus nerve to the glands of the 
stomach. Division of this nerve is followed by a cessation in the flow 
of the Juice. Stimulation of the peripheral end with induced electric 
currents at the rate of one or two per second causes the juice to be 
discharged. Nerve impulses therefore, discharged by this center descend 
the vagus nerve fibers to the glands and excite them to action. 

The production and discharge of the gastric juice just preceding and 
during a meal is the result of the action of two different stimuli, a primary 
and a secondary. 

The primary stimulus to gastric secretion, is a psychic state induced, 
on the one hand, by the sight or the odor of food especially if an indi- 
vidual is hungry and the food appetizing; and on the other hand by the 
mastication of food which is agreeable. The juice thus secreted is known 
as psychic or appetite juice. The quantity of the juice secreted will be 
proportional to the agreeable character of the psychic state and the 
thoroughness of mastication. As a result of the psychic states nerve 
impulses descend nerve fibers to the center in the medulla and excite it 
to increased activity. 

The secondary stimulus to the gastric secretion is in all probability 
chemic in character and developed in the stomach or in its walls during 
digestive activity, inasmuch as the secretion takes place independent of 
nerve influences and after division of all afferent and efferent nerves that 
pass from and to the stomach. The results of experiments indicate that 
there is produced in the gastric mucous membrane of the pyloric portion 
of the stomach some chemic agent, which is absorbed into the blood and 
carried to the glands throughout the stomach. On reaching the glands 
this agent excites them to continuous activity. For this reason the agent 
has been termed the gastric hormone or gastric secretin. The stimulus to 
the production of the hormone is believed to be either the action of certain 
articles of food, e.g., dextrin, meat broth or the first products of digestion. 

Physiologic Action of Gastric Juice. — The principal action of the gastric 
juice is the transformation of the different protein principles of the food 
into peptones, the intermediate stages of which are due to the influence 
of the acid and pepsin respectively. As soon as any one of the proteins is 
penetrated by the acid it is converted into acid-protein, a fact which in- 
dicates that the first step in gastric digestion is the acidification of the 
proteins. This having been accomplished, the pepsin becomes operative 
and in a varying length of time transforms the acid-protein into a new 
form of protein termed peptone. In this transformation it is possible 
to isolate intermediate bodies by the addition of ammonium and mag- 
nesium sulphates, to which the term proteoses has been given. Because 



72 HUMAN PHYSIOLOGY 

of the order in which they are obtained they have been divided into pri- 
mary and secondary. This supposed change is represented by the 
following scheme: 

Protein — Acid-protein — Proteose — Proteose — Peptone 

(pri mary) — (secondary) 

Peptones. — Peptones are the final products of the digestion of protein 
bodies in the stomach and differ from the bodies from which they are 
derived in the following particulars: 

i. They are diffusible — i.e., capable of passing readily through animal 
membranes. 

2. They are soluble in water and in saline solution. 

3. They are non-coagulable by heat and nitric or acetic acids. They can 
be readily precipitated, however, by tannic acid, by bile acids, and by 
mercuric chlorid. 

The enzyme rennln, causes the caseinogen of milk to undergo a peculiar 
change before the acid and pepsin can convert it into peptone. This 
change consists in the cleavage of the caseinogen into a soluble protein 
and another body which combining with calcium salts forms casein. 
Casein then undergoes a chemic transformation similar to that of all 
other proteins. 

The enzyme lipase is believed to digest fat when in a finely emulsified 
state in a manner similar to the corresponding enzyme of the pancreatic 
juice. 

Movements of the Stomach. — During the period of gastric digestion the 
walls of the stomach become the seat of a series of movements, somewhat 
peristaltic in character, which serve not only to incorporate the gastric 
juice with the food, but also serve to eject the liquefied portions of the food 
into the intestine. 

After the entrance of the food both the cardiac and pyloric orifices are 
closed by the contraction of their sphincters. Within five minutes (in 
the cat) annular constrictions begin in the pyloric region which move peris- 
tal tically toward the pylorus. As digestion proceeds these constrictions 
or contractions become more frequent and more vigorous. The result is 
a trituration and liquefaction of the food. So soon as it is liquefied the 
pylorus relaxes and permits of its discharge into the intestine. The 
pylorus then closes and further preparation of food goes on. From time 
to time the pylorus relaxes to permit the discharge of prepared and lique- 
fied food until digestion is completed. The reason assigned for the relaxa- 
tion of the sphincter muscle is the presence of a sufficient amount of free 



DIGESTION 



73 



hydrochloric acid on the gastric side. The reason assigned for its contrac- 
tion after the discharge of food into the duodenum is the presence of the 
hydrochloric acid in this region. With its neutralization by the alkalies 
there present, its influence in causing contraction of the sphincter gradually 
diminishes. In the cardiac region there is an absence of peristalsis though 
the muscle wall is in a state of active tone. The fundus acts as a reservoir 
for food and delivers its contents to the pyloric region as rapidly as it is 
ready to receive them. 

Nerve Mechanism of the Stomach. — The muscle activities of the walls 
of the stomach as well as the activities of the sphincter muscles, viz., 
the sphincter cardiae and sphincter pylori, are in part inaugurated and 
modified from time to time by the nerve system. In a general way it 
may be stated that the necessary muscle tonus is due to the action of the 
vagus nerve. Division of this nerve is followed by a loss of tonus. Stimu- 
lation causes an augmentation in the vigor of the contractions of the 
pyloric musculature, of the cardiac and fundus musculature and of the 
sphincters; an inhibition of these movements is brought about by stimula- 
tion of the splanchnic nerves. 

The Duration of Gastric Digestion. — The length of time required for 
the digestion of a meal will depend largely on the quantity and the quality 
of the foods consumed. The relative digestibility of different articles of 
food was tested by Dr. Beaumont on a mass with a gastric fistula. The 
results of his observations were recorded in a table of which the following 
is an abstract. 

Table Showing the Digestibility of Various Articles of Food 






Hours Minutes 



Hours Minutes 



Eggs, whipped i 

Eggs, soft boiled 3 

Eggs, hard boiled 3 

Oysters, raw 2 

Oysters, stewed 3 

Lamb, broiled . 2 

Veal, broiled . 4 

Pork, roasted . . 5 

Beefsteak, broiled. 3 

Turkey, roasted.. 

Chicken, boiled ...... ; 

Chicken, fricasseed 

Duck, roasted. . . \ 



30 
30 



45 



Soup, barley, boiled . . 
Soup, bean, boiled.. . 
Soup, chicken, boiled . 
Soup, mutton, boiled. 

Sausage , 

Green corn, boiled.. . 

Beans, boiled t 

Potatoes, roasted.. . . 

Potatoes, boiled 

Cabbage, boiled 

Turnips, boiled 

Beets, boiled 

Parsnips, boiled 



n 
3 
3 
3 
3 
3 
2 
2 
3 
4 
3 
3 
2 



30 
20 
45 
30 
30 
30 
30 
30 
15 
30 



74 HUMAN PHYSIOLOGY 

INTESTINAL DIGESTION 

The physical and chemic changes which the food principles undergo in 
the small intestine, and which collectively constitute intestinal digestion, 
are complex and probably more important than those taking place in the 
stomach, for the food is, in this situation, subjected to the solvent action of 
the pancreatic and intestinal juices, as well as to the action of the bile, 
each of which exerts a transforming influence on one or more substances 
and still further prepares them for absorption into the blood. 

To rightly appreciate the physiologic actions of the digestive juices 
poured into the intestine, the nature of the partially digested food as it 
comes from the stomach must be kept in mind. This consists of water, 
inorganic salts, acidified proteins, proteoses, peptones, starch, maltose, 
liquefied fat, saccharose, lactose, dextrose, cellulose, and the indigestible 
portion of meats, cereals, and fruits. Collectively they are known as 
chyme. As this acidified mass passes through the duodenum its contained 
acids excite a secretion and discharge of the intestinal fluids: e.g., pan- 
creatic juice, bile, and intestinal juice. Inasmuch as these fluids are 
alkaline in reaction they exert a neutralizing and precipitating influence on 
various constituents of the chyme. As soon as this has taken place, 
gastric digestion ceases and those chemic changes are inaugurated which 
eventuate in the transformation of all the remaining undigested nutritive 
materials into absorbable and assimilable compounds which collectively 
constitute intestinal digestion. 

The Small Intestine. — This portion of the alimentary canal is a convo- 
luted tube, measuring about seven meters ki length and 3.5 cm. in diame- 
ter, and extending from the pyloric orifice of the stomach to the beginning 
of the large intestine. 

The Walls of the Small Intestine. — The walls of the intestine consist of 
four coats: viz., serous, muscle, submucous, and mucous. 

1. The serous coat is the most external and is formed by a reflection oi 
the general peritoneal membrane. 

2. The muscle coat surrounds the entire intestine and consists of two 
layers of fibers: 1. an external or longitudinal, and 2. an internal or cir- 
cular. The longitudinal fibers form a thin layer all over the intestine. 
The circular fibers are much more numerous and completely surround 
the intestine throughout its entire extent. 

3. The mucous coat is soft and velvety and is covered by a single layer of 
columnar epithelial cells. Its entire surface presents small conical pro- 
jections termed villi. 



DIGESTION 75 

Blood-vessels, Nerves, and Lymphatics. — The blood-vessels of the small 
intestine, which are very numerous, are derived mainly from the su- 
perior mesenteric artery. After penetrating the intestinal walls the 
smaller vessels ramify in the submucous coat and send branches to the 
muscle and mucous coats, supplying all their structures with blood. 
After circulating through the capillary vessels the blood is returned by 
small veins which subsequently unite to form the superior mesenteric vein, 
which, uniting with the splenic and gastric veins, forms the portal vein. 
The nerve elements in the intestinal wall consist of two plexuses, one 
(Auerbach's) lying between the muscle coats, the other (Meissner's) 
lying in the submucous coat. To this nerve net, composed of nerve cells 
and nerve processes, found in connection with the muscle coats of the 
stomach, of the small and of the large intestine as well, the term myenteric 
plexus has been given. The lymphatics, which originate in the mucous 
and muscle coats, are very abundant. They unite to form those vessels 
seen in the mesentery and empty into the thoracic duct. 

Intestinal Glands. — The gland apparatus of the intestine by which the 
intestinal juice is secreted consists of the duodenal (Brunner's) and the 
intestinal (Lieberkuhn's) glands. 

The duodenal glands are situated beneath the mucous membrane and 
open by a short wide duct on its free surface. They are racemose glands 
lined by nucleated epithelium. The secretion of these glands is clear, 
slightly viscid, and alkaline. Its chemic composition and functions are 
unknown. 

The intestinal glands or follicles are distributed throughout the entire 
mucous membrane in enormous numbers. They are formed mainly by 
an inversion of the mucous membrane and hence open on its free surface. 
Each tubule consists of a thin basement membrane lined by a layer of 
spheric epithelial cells, some of which undergo distention by mucin and 
become converted into mucous or goblet cells. The epithelial secreting 
cells consist of granular protoplasm containing a well-defined nucleus. 
The intestinal follicles constitute the apparatus which secretes the chief 
portion of the intestinal juice. 

The Pancreas. — This gland is long, narrow and flattened and is situated 
deep in the abdominal cavity, lying just behind the stomach. It measures 
from fifteen to twenty centimeters in length, six in breadth, and two and a 
half in thickness. It is usually divided into a head, body, and tail. 

The pancreas communicates with the intestine by means of a duct. 
This duct commences at the tail and runs transversely through the body 
of the gland. As it approaches the head of the gland it gradually increases 



7 6 



HUMAN PHYSIOLOGY 



in size until it measures about two or three millimeters in diameter. It 
then curves downward and forward and opens into the duodenum. In 
its course through the gland it receives branches which enter it nearly at 
right angles. 

The pancreas is similar in structure to the salivary glands, and con- 
sists of the system of ducts terminating in acini. The acini are tubular 
or flask-shaped, and consist of a basement membrane lined by a layer 
of cylindric, conic cells, which encroach upon the lumen of the acini. 
The cells exhibit a difference in their structure (Fig. 8), and may be said 
to consist of two zones — viz., an outer parietal zone, which is transparent 





Fig. 8. — One Saccule of the Pancreas of the Rabbit in Different States 
of Activity. — (After Kiihne and Lea.) 

A. After a period of rest, in which case the outlines of the cells are indistinct and 
the inner zone — i.e., the part of the cells (a) next the lumen (c) — is broad and filled 
with fine granules. B. After the gland has poured out its secretion, when the cell 
outlines (d) are clearer, the granular zone (a) is smaller, and the clear outer zone is 
wider. 



and apparently homogeneous, staining rapidly with carmin; an inner 
zone, which borders the lumen, and is distinctly granular and stains but 
slightly with carmin. These cells undergo changes similar to those 
exhibited by the cells of the salivary glands during and after active secre- 
tion. As soon as the secretory activity of the pancreas is established, the 
granules disappear, and the inner granular layer becomes reduced to a 
very narrow border, while the outer zone increases in size and occupies 
nearly the entire cell. During the intervals of secretion, however, the 
granular layer reappears and increases in size until the outer zone is re- 
duced to a minimum. It would seem that the granular matter is formed 
by the nutritive processes occurring in the gland during rest, and is dis- 
charged during secretory activity into the ducts, and takes part in the 
formation of the pancreatic secretion. 



DIGESTION 77 

Toward the outer extremity of the pancreas there are found among the 
acini collections of globular cells arranged in rods or columns separated 
by connective tissue. They have been termed after their discoverer, the 
Islands of Langerhans. It is believed they produce an internal secretion 
which in some way regulates sugar metabolism. 

The Pancreatic Juice. — The pancreatic juice is transparent, colorless, 
strongly alkaline, and viscid, and has a specific gravity of 1,020. It is 
one of the most important of the digestive fluids, as it exerts a transform- 
ing influence upon all classes of alimentary principles, and has been 
shown to contain at least three distinct enzymes, viz., am yl ops in, 
trypsin, steapsin or lipase. It has the following composition: 

Composition of Pancreatic Juice 

Water 900 . 76 

Protein material 90 . 44 

Inorganic salts 8 . 80 



Mode of Secretion. — The secretion and discharge of the pancreatic 
juice is associated with the introduction of food into the mouth and 
stomach and its early passage into the duodenum and is brought about 
by the action of a primary and a secondary stimulus. 

The primary stimulus is a psychic state according to Pavlov induced 
by the sight, odor and taste of food and w T hich leads to the discharge of 
nerve impulses from nerve-cells in the medulla oblongata and their trans- 
mission by efferent nerves in the trunk of the vagus nerve, to the cells of 
the acini. It is probable that the impressions made by the food on the 
criminal filaments of the afferent fibers in the vagus nerve develop nerve 
impulses which, when transmitted to the medulla, occasion the discharge 
of nerve impulses that not only excite the secretion but increase the blood- 
supply as well. 

The secondary stimulus is chemic in character and developed in the 
glands of the mucous membrane of the duodenum by the action of the 
acids of the chyme, that is, of the digested foods, coming through the 
pylorus. 

If an extract of the glandular portion of the duodenal mucous membrane, 
made with hydrochloric acid 0.4 per cent, is injected into the blood it 
evokes a profuse discharge of pancreatic juice. As hydrochloric acid 
alone will not produce this effect it is assumed that the extract contains 
an agent that excites or arouses the pancreas to secretor activity and 
to which, therefore, the name secretin is given. The secretin developed 



78 HUMAN PHYSIOLOGY 

by the passage of the acid food over the surface of the mucous membrane 
is absorbed into the blood and carried eventually to the pancreas and 
brought into relation with the cells on which it exerts its stimulating 
action. This agent belongs to the class of hormones. 

Physiologic Action. — By virtue of its contained enzymes pancreatic 
juice acts on: 

i. On Starch. — When normal pancreatic juice or a glycerin extract of 
the gland is added to a solution of hydrated starch, the latter is speedily 
transformed into maltose, passing through the intermediate stage of 
dextrin. The process is in all respects similar to that observed in the 
digestion of starch by saliva. Pancreatic juice, however, is more energetic 
in this respect than saliva. The enzyme which effects this change is 
termed aniylopsin. When the starch which escapes salivary digestion 
passes into the small intestine and mingles with pancreatic juice, it is 
very promptly converted into maltose by the action or in the presence 
of this enzyme. 

2. On Protein. — The protein bodies which escape digestion in the 
stomach are converted into peptones by the action of the alkali and 
ferment. The first effect of the alkali is to change the protein into an 
alkali-protein, a. fact which indicates that in the digestion of protein by 
pancreatic juice, the first stage is alkalinization. This having been 
accomplished, the ferment trypsin transforms the alkali-albumin into 
peptone. The addition of magnesium sulphate to the digesting mixtures 
causes a precipitation of an intermediate termed proteose. For this 
reason it is believed that here also peptones are preceded in their develop- 
ment by proteoses, of which there is probably, however, but one form, 
viz., secondary proteoses. Long-continued action of the pancreatic juice, 
decomposes the peptone into leucin, tyrosin, histidin aspartic acid, etc., 
compounds which belong to the group of bodies known as amino-acids, 
etc. 

3. On Fat. — If pancreatic juice be added to a perfectly neutral fat — 
olein, palmitin, or stearin — and kept at a temperature of about ioo°F. 
(38°C), it will at the end of an hour or two be partially decomposed into 
glycerin and the particular fat acid indicated by the name of the fat 
used — e.g., oleic, palmitic, stearic. The oil will then exhibit an acid 
reaction. The reaction is represented in the following formula: 

C 3 H 5 (Ci8H3302)3 + 3H2O = C18H34O2 + C3H 6 (HO) 3 

Triolein. Water. Oleic Acid. Glycerin. 

If to this acidified oil there be added an alkali, e.g., potassium or 
sodium carbonate, the latter will at once combine with the fat acid to 



DIGESTION 79 

form a salt known as a soap. The reaction is expressed in the follow- 
ing equation: 

Na 2 C0 3 + C18H34O2 = 2NaCi8H 3 30 2 + H2CO3 
Sodium Carbonate. Oleic Acid. Sodium Oleate. Carbonic Acid. 

Coincident with the formation of the soap, the remaining portion of the 
neutral oil will undergo division into globules of microscopic size, which 
are held in suspension in the soap solution, forming what has been termed 
an emulsion, which is white and creamy in appearance. The cause of 
this minute subdivision of the fat and the necessity for it is unknown. 
It may be assumed that by virtue of the subdivision a greater surface is 
exposed to the action of the pancreatic enzyme and the digestion of the 
fat thereby facilitated. The action of the pancreatic juice may then be 
said to consist in the cleavage of the neutral fats into fatty acids and 
glycerin, after which the formation of the soap and the division of the 
fat takes place spontaneously. The enzyme which produces the cleavage 
of the neutral fats has been termed steapsin or lipase. 

Physiologic Action of Intestinal Juice. — By reason of its contained en- 
zymes intestinal juice acts: 

1. On Proteoses and Peptones. — These bodies were supposed at one time 
to represent the final stages in the digestion of the proteins. This view 
is no longer entertained. It is now generally believed that under the 
influence of the intestinal juice they undergo a disruption into very simple 
bodies, known as amino-acids. This disruption is brought about by an 
agent termed erepsin. Inasmuch as the long-continued action of pan- 
creatic juice also disrupts the peptones, it is also believed to contain 
erepsin. 

2. On The Compound Sugars. — Saccharose, maltose and lactose, the 
three compound sugars, are believed by most observers to be not only non- 
absorbable, but also non-assimilable and, therefore, are required to undergo 
some digestive change before they can be absorbed and assimilated. 
An extract of the intestinal mucous membrane or the intestinal juice 
of the dog added to a solution of saccharose will cause it to combine 
chemically with water after which a cleavage into dextrose and levulose 
will take place, which together constitute invert sugar. The enzyme 
to which this action is attributed has been termed invertase or saccharase. 
Maltose undergoes a similar change. After its combination with water 
it undergoes a cleavage into two molecules of dextrose. Lactose appears 
to be unaffected by the pure juice. As it is non-assimilable it has been 
supposed to undergo conversion into dextrose and galactose while passing 
through the epithelial cells of the intestinal mucosa. In either case the 



80 HUMAN PHYSIOLOGY 

transformation is brought about by two ferments known respectively as 
maltase and lactase. 

3. On Trypsinogen. — This zymogen when first discharged from the pan- 
creatic duct is inactive and incapable of effecting the necessary digestive 
changes in the proteins. Shortly after its entrance into the intestine, it 
becomes quite active and efficient, a change attributed to an agent 
entero-kinase secreted by the mucosa in the upper part of the intestine. 

The Bile. — This fluid is a product of the secretor activity of the liver- 
cells. After its formation by the liver cells it is conveyed from the liver 
by the bile capillaries which unite finally to form the main hepatic duct. 
This duct emerges from the liver at the transverse fissure. At a distance 
of about 5 centimeters it is joined by the cystic duct, the distal extremity 
of which expands into a pear-shaped reservoir, the gall-bladder in which 
the bile is temporarily stored. The duct formed by the union of the 
hepatic and cystic ducts, the common bile-duct, passes downward and 
forward for a distance of about 7 centimeters, pierces the walls of the 
intestine and passes obliquely through its coats for about a centimeter and 
opens into a small receptacle, the ampulla of Vater. 

Physical Properties. — The bile coming from the liver is thin and watery. 
That obtained from the gall-bladder is more or less viscid from the presence 
of mucin. The specific gravity of human bile varies within normal 
limits from 1.010 to 1.020. The reaction is invariably alkaline in the 
human subject when first discharged from the liver, but may become 
neutral in the gall-bladder. The alkalinity depends on the presence of 
sodium carbonate and sodium phosphate. When fresh, it is inodorous; 
but it readily undergoes putrefactive changes, and soon becomes offensive. 
Its taste is decidedly bitter. When shaken with water, it becomes frothy 
— a condition which lasts for some time and which is due to the presence 
of mucin. In ox bile the mucin is replaced by a nucleo-protein. 

The color of bile obtained from the hepatic duct is variable, usually a 
shade between a greenish yellow and a brownish red. In different 
animals the color varies. In the herbivorous animals it is usually green; 
in the carnivorous animals it is orange or brown. In man it is green or a 
golden yellow. The colors are due to the presence of pigments. Micro- 
scopic examination fails to show the presence of structural elements. 

Chemic Composition. — Human bile obtained from an accidental biliary 
fistula was shown by Jacobson to contain the following ingredients, viz. : 



DIGESTION 8l 

Composition of Human Bile 

Water 977 .40 

Sodium glycocholate 9-94 

Sodium taurocholate a trace 

Cholesterin 0.54 

Free fat o . 10 

Sodium palmitate and stearate 1 .36 

Lecithin 0.04 

Organic matter, and pigments bilirubin and biliverdin 2.26 

Inorganic salts 8 . 36 



1,000.00 



Sodium glycocholate and sodium taurocholate are the characteristic 
biliary salts. They are compounds of sodium and glycocholic and 
taurocholic acids. There is evidence that the former is formed by the 
union of an amino-acid glycocoll and cholic acid, and that the latter is 
formed by the union of taurin, a derivative of the amino-acid 
cystin, both of which are absorbed from the intestines. The origin of 
cholic acid is not clear. 

There is good evidence for the view, that after their discharge into 
the intestine, the bile salts are absorbed, with the exception of a portion 
destroyed by bacteria, and carried by the portal vein to the liver and again 
excreted. By this circulation from liver to intestine and from intestine to 
the liver, the work of the liver cells in the synthesis or secretion of bile 
acids, is supposed to be reduced to a minimum. It is also probable that a 
portion of the acids enters the general circulation and influences favorably 
the general nutrition. It is stated by some investigators that the activities 
of the liver cells are decidedly increased by the circulation of the bile salts 
and that they are to be regarded as the natural stimuli to the secretion. 

Cholesterin. — Cholesterin when obtained from bile presents itself in the 
form of flat rectangular crystals. Though a constant constituent of 
bile, is not confined to this fluid as it has been shown to be a normal 
constituent of all animal and vegetable cells, though it is particularly 
abundant in the myelin of nerve-fibers. Though cholesterin has for a 
long time been regarded merely as one of the products of the katabolism 
of living material, it has come to be believed that it is necessary to the 
vitality of tissue cells and especially to the blood cells. Entering into 
the composition of the surface layer of cells, it prevents the entrance of 
certain toxins which would have a destructive influence on their structure 
or composition. In the metabolism of cells it is set free after which it 
ts into the blood to be secreted by the liver. In the bile it frequently 
undergoes crystallization and forms one of the forms of gall-stones. In 
6 



82 HUMAN PHYSIOLOGY 

the bile the cholesterin is held in solution by the biliary salts. In the I 
intestine it is converted into stercorin and discharged in the feces. 

Bilirubin, Biliverdin. — These two pigments impart to the bile its 
red and green colors respectively. Bilirubin is present in the bile of human 
beings and the carnivora, biliverdin in the bile of the herbivora. As the 
former pigment readily undergoes oxidation in the gall-bladder, giving rise 
to the latter pigment, almost any specimen of bile may present any shade 
of color between red and green. Bilirubin is regarded as a derivative of 
hematin, one of the cleavage products of hemoglobin, the coloring-matter 
of the blood. In the liver the hematin combines with water, loses its iron 5 
and is changed to bilirubin. By continuous oxidation there are formed 
biliverdin, bilicyanin, and choletelin. After their discharge into the in- 
testine the bile pigments are finally reduced to hydrobilirubin or an allied 
substance, stercobilin, which becomes one of the constituents of the feces 
A portion of the latter is absorbed into the blood and ultimately discharged 
into the urine where it is known as urobilin. 

Lecithin, — Lecithin is regarded, because of its physical properties and . 
chemic composition, as a complex fat. When pure it presents itseli 
generally as a white crystalline powder, though very frequently as a white 
waxy mass which is soluble in ether and alcohol. Lecithin is widely dis- 
tributed throughout the body, being found in blood, lymph, red and 
white corpuscles, nerve- tissue, yolk of egg, semen, milk, and bile. Lecithir 
has been regarded as one of the decomposition products of nerve-tissue 
removed from the blood by the liver and thus becoming one of the con- \ 
stituents of the bile, in which it is held in solution by the bile salts 
Lecithin can be readily decomposed by various agents yielding gly- 
cophosphoric acid, a fat acid and cholin. 

The Mode of Secretion and Discharge of Bile. — The flow of bile frorr 
the liver is continuous but subject to considerable variation during the 
twenty-four hours. The introduction of food into the stomach at once 
causes a slight increase in the flow, but it is not until about two hours latei 
that the amount discharged reaches its maximum; after this period il 
gradually decreases up to the eighth hour, but never entirely ceases 
During the intervals of digestion though a small quantity passes into the 
intestine, the main portion is diverted into the gall-bladder, because o)> 
the closure of the common bile-duct by the sphincter muscle near its 
termination, where it is retained until required for digestive purposes 
When acidulated food passes over the surface of the duodenum, there is ar 
increase in the secretion or at least the discharge of bile, due to the relaxa- 
tion of the sphincter muscle of the common bile-duct and the contractior 
of the muscle walls of the gall-bladder and biliary passages. 



DIGESTION 83 

Physiologic Action of Bile. — The exact relation of the bile to the digest- 
ive process has not been satisfactorily determined. No specific action 
can be attributed to it. It has but a slight, if any, diastatic action on 
starch. It is without influence on proteins or on fats directly. But 
indirectly and by virtue of the bile salts it contains, it plays an important 
part in increasing the action of the pancreatic enzymes. Thus the anxio- 
lytic or starch transforming power of the pancreatic juice is almost doubled 
and the same is true for its proteolytic power, while its lipolytic or fat- 
splitting power is tripled. 

The bile salts also dissolve insoluble soaps which may be formed during 
digestion and thus favors the digestion of fat. If it be excluded from the 
intestine there is found in the feces from 22 to 58 per cent, of the ingested 
fats. At the same time the chyle, instead of presenting the usual white 
creamy appearance, is thin and slightly yellow. The manner in which the 
bile promotes fat digestion is yet a subject of investigation. If all the fat 
is converted into fat acids and glycerin, with the formation of soaps, as 
seems probable, the action of the bile becomes more apparent from the fact, 
ilready stated, that it dissolves and holds in solution the soaps so formed 
ffhich would be necessary to their absorption by the epithelial cells. This 
iction has been attributed to the presence of the bile salts. As an aid to 
digestion the bile has been regarded as important, for the reason that its 
entrance into the intestine is attended by a neutralization and precipita- 
.ion of the proteins which have not been fully digested and are yet in the 
.tage of acid-albumin. In this way gastric digestion is arrested and the 
oods are prepared for intestinal digestion. 

Though bile possesses no antiseptic properties outside the body, itself 
mdergoing putrefactive changes very rapidly, it has been believed that in 
he intestine it in some way prevents or retards putrefactive changes in the 
ood. There can be no doubt that if the bile is prevented from entering 
he intestine there is an increase in the formation of gases and other prod- 
icts which impart to the feces certain characteristics which are indicative 
•f putrefaction. As to the manner in which bile retards this process 
nothing definite can be stated. It has been supposed to be a stimulant 
,0 the peristaltic movements of the intestine, inasmuch as these move- 
nents diminish when bile is diverted from the intestine. 

Intestinal Movements. — During intestinal digestion the walls of the 
itestine exhibit two kinds of movement, viz., a rhythmic segmentation 
nd a peristalsis. By the former the food is divided into segments and 
•y the latter, it is carried down the intestine. Shortly after the entrance 
f food into the duodenum a broad peristaltic wave promptly carries it 
ownward a variable distance a rhythmic segmentation begins by a con- 



84 HUMAN PHYSIOLOGY 

traction of bands of circular muscle fibers. So soon as a mass of food is 
divided into segments each segment is in turn again divided by similar 
contractions. The lower half of each segment then unites with the upper 
half of the segment below to commingle with it and to expose new surfaces 
of the food mass to contact with the intestinal juices and to the mucous 
membrane. A continual repetition of this process results in a thorough 
mixing of the food with the digestive juices. Subsequent peristaltic 
waves slowly carry the food further down the intestine, after which a 
further segmentation takes place. These alternate movements continue 
throughout the digestive process. 

The Nerve Mechanism of the Small Intestine. — The rhythmic segmen- 
tation movements are the result of an intraintestinal pressure due to the 
accumulation of food, provided the intestinal walls possess the requisite 
degree of tonicity. The tonicity is imparted to the muscle coat by nerve 
impulses coming from the central nerve system through the efferent vagus 
nerve fibers. The orderly and coordinated contractions and relaxations 
of the muscle coat which constitute a peristaltic movement are mediated 
by the myenteric plexus — the nerve plexus of Meissner and Auerbach — 
and therefore termed a myenteric reflex. 

The intestine is connected with the central nerve system by the vagi 
and splanchnic nerves, both of which influence the tonus and the vigor 
of the intestinal contractions in one direction or the other. Thus stimula- 
tion of the vagus nerve increases the contractions, while stimulation of 
the splanchnic inhibits the contractions. The degree of activity of the 
intestine at any one moment is the resultant of the opposing actions of 
these two nerves. 

The Large Intestine. — The large intestine is that portion of the ali- 
mentary canal situated between the termination of the ileum and the 
anus. It varies in length from one and a quarter to one and a half meters, 
in diameter from three and a half to seven centimeters. It is divided into 
the cecum, the colon (subdivided into an ascending, transverse, and 
descending portion, including the sigmoid flexure), and the rectum. 

The walls of the large intestine consist of three coats: viz., serous, 
muscular, and mucous. 

The serous is a reflection of the general peritoneal membrane. 

The muscle is composed of both longitudinal and circular fibers. The 
longitudinal fibers are collected into three narrow bands which are situated 
at points equidistant from one another. At the rectum they spread out 
so as to surround it completely. As the longitudinal bands are shorter 
than the intestine itself, its surface becomes sacculated, each sac being 



DIGESTION 85 

partially separated from adjoining sacs by narrow constrictions. The cir- 
cular fibers are arranged in the form of a thin layer over the entire intes- 
tine. Between the sacculi, however, they are more closely arranged. The 
sacculi have been termed haustra from their supposed function, that of 
absorbing or drawing water from the intestinal contents thus imparting 
to them a certain degree of consistency. In the rectum the circular fibers 
are well developed, and at a point an inch above the anus they form, as 
stated above, the internal sphincter. 

The mucous membrane of the large intestine possesses neither villi nor 
valvule conniventes. It contains a large number of tubules consisting 
of a basement membrane lined by columnar epithelium. They resemble 
the follicles of Lieberkiihn. The secretion of these glands is thick and 
viscid and contains a large quantity of mucin. 

The Movements of the Large Intestine. — After the absorption of the 
prepared food materials, the remaining contents of the intestine, together 
with certain intestinal secretions and the excrementitioas matter of the 
bile, pass into the large intestine and assist in the formation of the feces. 

Under the influence of a peristaltic movement similar to that wit- 
nessed in the small intestine, all this excrementitious matter, deprived by 
absorption of the excess of its contained water and nutritive material, is 
gradually carried downward to the sigmoid flexure, where it accumulates 
prior to its extrusion from the body. The effects of the peristaltic waves 
are to some extent interfered with by anti- peristaltic or anastaltic waves 
which, beginning in the transverse colon, run toward and to the cecum. 
An antiperistaltic wave occurs in the cat about every fifteen minutes and 
lasts for about five minutes. The intestinal contents are thereby driven 
back toward the cecum. The effect is a still further admixture with the 
secretions and exposure to the absorbing mucosa. 

The Function of the Large Intestine. — The function of the large intes- 
tine is therefore to receive, to reduce to a proper consistency, to tempo- 
rarily store and subsequently discharge its contents, consisting of the indi- 
gestible residue of the food, together with excretions of intestinal glands 
which have descended from the small intestine and which constitute in 
part the feces. 

Intestinal Fermentation.- Owing to the favorable conditions in both 
the small and large intestine for fermentative and putrefactive processes — 
e.g., heat, moistur* 1. ;ind the presence of various microorganisms — 

the I 11 < onsumed in < quantity or when acted on by defect- 

ive . undergi t decomposition changes which arc- 

attended by the production of gases and various chemic compounds. 



86 HUMAN PHYSIOLOGY 

Among the more important of these compounds may be mentioned indol, 
skatol, cresol and phenol. They arise from the putrefactive decomposi- 
tion of various amino-acids. A certain portion of each is eliminated in 
the feces while another portion is absorbed into the portal circulation, 
oxidized, and combined with potassium sulphate. By this means their 
toxicity is destroyed. They are subsequently eliminated in the urine. 
The amount of the potassium indoxyl sulphate or indican in the urine is 
taken .as a measure of the extent of intestinal putrefaction. 

The Feces. — The feces is a term applied to the mass of material ejected 
from the rectum through the anus. They are characterized by consist- 
ency, color and odor, properties which are connected with the rapidity 
with which they are carried through the intestine, the quality of the food, 
and the extent of the fermentative and putrefactive changes they undergo. 

Defecation. — Defecation is the voluntary act of extruding the feces 
from the rectum, and is accomplished by a relaxation of the sphincter ani 
muscle and by the contraction of the muscular walls of the rectum, aided 
by the contraction of the abdominal muscles. 

ABSORPTION 

The term absorption is applied to the passage or transference of 
materials into the blood from the tissues, from the serous cavities, and from 
the mucous surfaces of the body. The most important of these surfaces, 
especially in its relation to the formation of blood, is the mucous surface 
of the alimentary canal; for it is from this organ that new materials are 
derived which maintain the quality and quantity of the blood. The ab- 
sorption of materials from the interstices of the tissues is to be regarded 
rather as a return to the blood of liquid nutritive material which has es- 
caped from the blood-vessels for nutritive purposes, and which, if not 
returned, would lead to an accumulation of such fluid and the development 
. of dropsical conditions. 

The anatomic mechanisms involved in the absorptive processes are, 
primarily, the lymph-spaces, the lymph-capillaries , and the blood-capillaries; 
secondarily, the lymphatic vessels and larger blood-vessels. 

Lymph-spaces, Lymph-capillaries, Blood-capillaries. — Everywhere 
throughout the body, in the intervals between connective-tissue bundles 
and in the interstices of the several structures of which an organ is com- 
posed, are found spaces of irregular shape and size, determined largely by 
the nature of the organ in which they are found, which have been termed 
lymph-spaces or lacuna, from the fact that during the living condition 



ABSORPTION 87 

they are continually receiving the lymph which has escaped from the blood- 
vessels throughout the body. In addition to the connective-tissue 
lymph-spaces, various observers have described special lymph-spaces in 
the testicle, kidney, liver, thymus gland, and spleen; in all secreting glands 
between the basement membrane and blood-vessels; around blood-vessels 
(perivascular spaces), and around nerves. The serous cavities of the body 
— peritoneal, pleural, pericardial, etc. — may also be regarded as lymph- 
spaces, which are in direct communication by open mouths or stomata 
with the lymph capillaries. This method of communication is not only 
true of serous membranes, but to some extent also of mucous membranes. 
The cylindric sheaths and endothelial cells surrounding the brain, spinal 
cord, and nerves can also be looked upon as lymph-spaces in connection 
with lymph-capillaries. 

The blood-capillaries not only permit the passage of the liquid nutritive 
portions of the blood across their delicate walls, but are also engaged in 
the reabsorption of this transudate, as well as in the absorption of new 
materials from the alimentary canal. The extensive capillary network 
which is formed by the ultimate subdivision of the arterioles in the sub- 
mucous tissue and villi of the small intestine forms an anatomic arrange- 
ment well adapted for absorption. It is now well known that in the ab- 
sorption of the products of digestion the blood-capillaries are more active 
than the lymph-capillaries. 

The Blood-vessels. — The blood-vessels which are concerned in the 
conduction of fresh nutritive material from the alimentary canal have 
their origin in the elaborate capillary network in the mucous membrane. 
The small veins which emerge from the network gradually unite, forming 
larger and larger trunks, which are known as the gastric, superior, and 
inferior mesenteric veins. These finally unite to form the portal vein, a 
short trunk about three inches in length. The portal vein enters the liver 
at the transverse fissure, after which it forms a fine capillary plexus 
ramifying throughout the substance of the liver; from this plexus the 
hepatic veins take their origin, and finally empty the blood into the vena 
cava inferior. (See Fig. 9.) 

The lymph-capillaries, in which the lymph-vessels proper take their 
:n, arearranged in the form of plexuses of quite irregular shape. In 
most situations they are intimately interwoven with the blood-vessels, 
from which, however, they can be readily distinguished by their larger 
caliber and irregular expansions. The wall of the lymph-capillary is 
formed by a single layer of epithelioid cells, with sinuous outlines, and 
which accurately dove- tail with one another. In no instance are valves 
found. In the villus of the small intestine the beginning of the lymphatic 



88 



HUMAN PHYSIOLOGY 



is to be regarded as a lymph-capillary, generally club-shaped, which at 
the base of the villus enters a true lymphatic; at this point a valve is 
situated, which prevents regurgitation. The lymph capillaries anastomose 
freely with one another, and communicate on the one hand with the lymph- 
spaces, and on the other with the lymphatic vessels proper. 




Fig. 9. — Diagram of the portal vein (pv) arising in the alimentary tract and spleen 
(s) and carrying the blood from these organs to the liver. — (Yeo's "Text-book of 
Physiology.") 

As the shape, size, etc., of both lymph-spaces and capillaries are deter- 
mined largely by the nature of the tissues in which they are contained, it is 
not always possible to separate the one from the other. Their function, 
however, may be regarded as similar — viz., the collection of the lymph 
which has escaped from the blood-vessels, and its transmission onward 
into the regular lymphatic vessels. 

Lymph-vessels. — These constitute a system of minute, delicate trans- 
parent vessels, found in nearly all the organs and tissues of the body. 
Having their origin at the periphery in the lymph- capillaries and spaces, 



AB SORPTION 89 

they rapidly converge toward the trunk of the body and empty into the 
thoracic duct. In their course they pass through numerous small ovoid 
bodies, the lymphatic glands. 

The lymph-vessels of the small intestines — the lacleals — arise within the 
villus processes which project from the inner surface of the intestine 
throughout its entire extent. The wall of the villus is formed by an 
elevation of the basement membrane, and is covered by a layer of columnar 
epithelial cells. The basis of the villus consists of adenoid tissue, a fine 
plexus of blood-vessels, unstriped muscle-fibers, and the lacteal vessel. 
The adenoid tissue consists of a number of intercommunicating spaces, 
containing leukocytes. The lacteal vessel possesses a thin but distinct 
wall composed of endothelial plates, with here and there openings which 
bring the interior of the villus into communication with the spaces of the 
adenoid tissue. 

The structure of the larger vessels resembles that of the veins, consisting 
of three coats: 

1. External, composed of fibrous tissue artd muscle fibers, arranged 
longitudinally. 

2. Middle, consisting of white fibers and yellow elastic tissue, non- 
striated muscle-fibers, arranged transversely. 

3. Internal, composed of an elastic membrane, lined by endothelial 
cells. 

Throughout their course are found numerous semilunar valves, opening 
toward the larger vessels, formed by a folding of the inner coat and 
strengthened by connective tissue. 

Lymph Glands. — The lymph glands consist of an external capsule 
composed of fibrous tissue which contains non-striped muscle-fibers; from 
its inner surface septa of fibrous tissue pass inward and subdivide the 
gland-substance into a series of compartments, which communicate with 
one another. The blood-vessels which penetrate the gland are sur- 
rounded by fine threads, forming a follicular arrangement, the meshes of 
which contain numerous lymph-corpuscles. Between the follicular 
threads and the wall of the gland lies a lymph-channel traversed by a 
reticulum of adenoid tissue. The lymph- vessels, after penetrating this 
ule, pour their lymph into this channel, through which it passes; it is 
then collected by the efferent vessels and transmitted onward. The 
lymph-corpuscles which are washed out of the gland into the Lymph- 
Stream arc forr probably, by division of preexisting cells. 

The Thoracic Duct. The thoracic dud is the general trunk of the 
lymphatic system; into it the vessels of the lower extremities, of the 



90 HUMAN PHYSIOLOGY 

abdominal organs, of the left side of the head, and of the left arm empty 
their contents. It is about fifty mm. in length, arises in the abdomen, 
opposite the third lumbar vertebra, by a dilatation (the receptaculum chyli) , 
ascends along the vertebral column to the seventh cervical vertebra, and 
terminates in the venous system at the junction of the internal jugular and 
subclavian veins on the left side. The lymphatics of the right side of the 
head, of the right arm, and of the right side of the thorax terminate in 
the right thoracic duct, about one inch in length, which joins the venous 
system at the junction of the internal jugular and subclavian on the right 
side. 

The general arrangement of the lymph vessels is shown in figure 10. 

Lymph. — Lymph is a clear colorless fluid found in the tissue spaces and 
in the lymph vessels. The former is termed intercellular, the latter intra- 
vascular. 

Physical Properties and Chemic Composition. — Lymph is clear, colorless, 
alkaline in reaction, saline to the taste and has a specific gravity varying 
from i. 020 to 1.030. It holds in suspension a number of corpuscles 
resembling in their general appearance the white corpuscles of the blood. 
Their number has been estimated at 8,200 per cubic millimeter, though the 
number varies in different portions of the lymphatic system. As the 
lymph flows through the lymphatic gland it receives a large addition of 
corpuscles. Lymph-corpuscles are granular in structure, and measure 
H. 500 of an inch in diameter. When withdrawn from the vessels, lymph 
undergoes a spontaneous coagulation similar to that of blood, after which 
it separates in serum and clot. 

Chemic analysis shows that it consists of water, proteins 3 to 4 per 
cent., fat 0.004 to 0.13 per cent., sugar, 0.10 to 0.15 per cent., inorganic 
salts, 0.8 to 0.9 per cent., urea CO2 and other katabolic products are 
present in small amounts. 

Origin and Functions of Intercellular Lymph. — Though the blood is 
the common reservoir of all nutritive materials, they are not available 
for nutritive purposes as long as they are confined within the blood- 
vessels. But owing to the character of the wall of the capillary blood- 
vessels, some of the constituents of the blood-plasma pass across it and 
are received by the tissue-spaces in which they come into contact with 
the tissue-cells. To the sum total of these materials the term lymph 
is given. The forces concerned in the passage of the constituents of the 
lymph across the capillary wall are diffusion, osmosis and nitration. Other 
forces have been suggested such as the secretory activity of the capillary 
cells and the increased concentration of the lymph due to the accumula- 



ABSORPTION 



91 




PlG. 10. — Diagram Showing the Course of the Main- Trunk of the Absorbent 
System. — (Yeo's "Textbook of Physiology.") 

The lymph-vessels of lower extremities (D) meet the lacteals of intestines (LAC 
at the receptaculum chyli (RC), where the thoracic duct begins. The superficial 
vessels are shown in the diagram on the right arm and leg (S), and the deeper ones 
on the arm to the left (D). The glands are here and there shown in groups. The 
small right duct opens into th* veins on the right Bide. The thoracic duct opens 
into the union of tne great veins of the left side of the neck (T). 



92 HUMAN PHYSIOLOGY 

tion of katabolic products whereby the osmotic pressure is increased. 
Its function becomes apparent from its origin and composition, its situa- 
tion and relation to the tissues. It is to furnish the tissue-cells with those 
nutritive materials which are necessary for their growth, repair and func- 
tional activity. It also receives all waste products that arise from their 
activity prior to their removal by the blood- and lymph- vessels. 

Absorption of Intercellular Lymph. — From the fact that lymph is being 
discharged more or less continuously from the thoracic duct, it is evident 
that lymph is being absorbed from the intercellular spaces; from which it 
may be inferred that more lymph is passing from the blood into the tissue- 
space than is necessary for the immediate needs of the tissues. To pre- 
vent an accumulation and an interference through pressure, with the 
activities of the tissues, the excess is absorbed by the lymph-vessels and 
returned to the blood stream by way of the thoracic duct. It is likely 
that some of the constituents are also absorbed by the blood-vessels. 

Absorption of Food. — Physiological experiments have demonstrated 
that the agents concerned in the absorption of new materials from the 
alimentary canal are: 

i. The blood-vessels of the entire canal, but more particularly those 
uniting to form the portal vein. 

2. The lymph vessels coming from the small intestine, which converge 
to empty into the thoracic duct. 

As a result of the action of the digestive fluids upon the different classes 
of food principles — proteins, sugars, starches, and fats — there are formed 
amino-acids, dextrose and levulose, soap and glycerin, which differ from 
the former in being highly diffusible — a condition essential to their 
absorption. 

Their absorption is accomplished by the villous processes covering the 
surface of the intestinal mucous membrane. 

The Villi. — The villi are small filiform or conical processes projecting 
from the surface of the mucous membrane. Each villus consists of a 
basement membrane supporting columnar epithelial cells. In the interior 
of the villus there is frame work of connective tissue supporting arteries, 
capillaries and veins and a single club-shaped lymph capillary. 

Function of the Villi. — The villi, and especially the epithelial cells 
covering them, are the essential agents in the absorption of the products 
of digestion. It is by the activity of these cells that the new materials 
are taken out of the alimentary canal and transferred into the lymph- 
spaces in the interior of the villi, from which they are subsequently 
removed by the blood-vessels and lymph- vessels. 



ABSORPTION 93 

The water and inorganic salts and sugars after their absorption by the 
epithelium of the villi pass onward into the interior of the villi; thence 
across the capillary wall into the blood by which they are carried to the 
liver. The water and salts in all probability pass directly through the 
liver to become part of the general blood volume. The sugar is in part 
removed from the blood stream and temporarily stored in the liver cells 
under the form of glycogen or liver starch. The products of protein 
digestion — the amino-acids — are also absorbed by the epithelial cells. 
It was until recently believed that during their transit through the cell 
they were synthesized into plasma-albumin which was then discharged 
into the blood stream. More recent experiments indicate that this is 
not the case and that the amino-acids pass directly into and are found 
circulating in the blood. 

The products of fat digestion — soap and glycerin — after absorption 
are synthesized to fat which is deposited in the epithelial cells in the form 
of small drops, after which it too passes to the interior of the villus to 
enter the lymph capillary. 

The products of digestion find their way into the general circulation by 
two routes: 

i. The icater, protein, dextrose, and soluble salts, after passing into the 
lymph-spaces of the villi, pass across the wall of the capillary blood- 
vessel; entering the blood, they are carried to the liver by the vessels 
uniting to form the portal vein, Fig. 9; emerging from the liver, they are 
emptied into the inferior vena cava by the hepatic vein. 

2. The fat enters the lymph-capillary in the interior of the villus; by the 
contraction of the layer of muscle-fibers surrounding it, its contents are 
forced onward into the lymph-vessels, thence into the thoracic duct, and 
finally into the blood stream at the junction of the internal jugular and 
subclavian veins on the left side, Fig. 10. 

Chyle. — Chyle is the fluid found in the lymph vessels, coming from the 
small intestine after the digestion of a meal containing fat. In the 
intervals of digestion the fluid of these lymphatics is identical in all respect 
with the lymph found in all other regions of the body. As soon as the 
granular fat passes into the lymph vessels and mingles with the lymph 
it becomes milky white in color, and the vessels which previously were in- 
visible become visible, and resemble white threads running between the 
layers of the mesentery. Chyle has a composition similar to that of 
lymph, but it contains, in addition, numerous fatty granules. When 
examined microscopically, the chyle presents a fine molecular basis, made 
up of the finely divided granules of fat. 



94 HUMAN PHYSIOLOGY 

Forces Aiding the Movement of Lymph and Chyle. — The lymph and 
chyle are continually moving in a progressive manner from the periphery 
or beginning of the lymphatic system to the final termination of the 
thoracic duct. The force which primarily determines the movement of 
the lymph has its origin in the beginnings of the lymph-vessels, and 
depends upon the difference in pressure here and the pressure in the 
thoracic duct. The greater the quantity of fluid poured into the lymph- 
spaces, the greater will be the pressure and, consequently, the move- 
ment. The first movement of chyle is the result of a contraction of the 
muscle-fibers within the walls of the villus. At the time of contraction 
the lymph capillary is compressed and shortened, and its contents are 
forced onward into the true lymphatic. When the muscle-fibers relax, 
regurgitation is prevented by the closure of the valve in the lymphatic at 
the base of the villus. 

As the walls of the lymph vessels contain muscle-fibers, when they 
become distended these fibers contract and assist materially in the onward 
movement of the fluid. 

The contraction of the general muscular masses in all parts of the body, 
by exerting an intermittent pressure upon the lymphatics, also hastens 
the current onward; regurgitation is prevented by the closure of valves 
which everywhere line the interior of the vessels. 

The respiratory movements aid the general flow of both lymph and chyle 
from the thoracic duct into the venous blood. During the time of an 
inspiratory movement the pressure within the thorax, but outside the 
lungs, undergoes a diminution in proportion to the extent of the move- 
ment; as a result, the fluid in the thoracic duct outside of the thorax, 
being under a higher pressure, flows more rapidly into the venous system. 
At the time of an expiration, the pressure rises and the flow is temporarily 
impeded, only to begin again at the next inspiration. 

THE BLOOD 

The blood may be defined as the nutritive fluid of the body since it 
contains all those materials that are necessary to the maintenance of the 
nutrition. The presence and proper circulation of the blood in the living 
organism are essential for the maintenance of tissue irritability and for 
the manifestation of the activities of all physiologic mechanisms. The 
escape of the blood from the vessels, especially in the higher animals, is 
followed by cessation of the physiologic activities of all the tissues within a 
short period. The irritability, however, persists for a variable length of 
time though it too gradually declines and finally disappears. The blood 






THE BLOOD 95 

is also a reservoir for the reception of katabolic products produced by 
and absorbed from the tissues. 

The Physical Constitution of Blood. — A microscopic examination of 
the blood as it flows through the capillary vessels of the web of the frog or 
the mesentery of the rabbit shows that it is not a homogeneous fluid, but 
that it consists of two distinct portions, viz.: (i) a clear, transparent, 
slightly yellow fluid, the plasma or liquor sanguinis: (2) small particles 
termed corpuscles floating in it, of which there are two varieties, the red 
or the erythrocytes and the white or the leukocytes. By appropriate methods 
it can be shown that a third corpuscle, colorless in appearance and smaller 
in size than the ordinary white corpuscle, is present in the blood stream 
and known as the blood-platelet or plaque. 

Physical Properties. — The color of the blood in the arteries is scarlet red, 
in the veins bluish red. The cause of the color is the presence of a col- 
oring matter, hemoglobin, in different degrees of combination with oxygen. 
As the venous blood passes into and through the pulmonic capillaries the 
hemoglobin absorbs a certain volume of oxygen after which it changes 
in color and on emerging from the lungs imparts to the blood its charac- 
teristic scarlet-red color. By reason of the union of the hemoglobin with 
the oxygen it is generally termed while in the arteries, oxyhemoglobin. 
As the arterial blood passes into and through the systemic capillaries, 
the oxyhemoglobin yields up a portion of its oxygen to the tissues after 
which it again changes in color and on emerging from the tissues imparts 
to the blood its characteristic bluish-red color. By reason of the loss of a 
portion of its oxygen, the hemoglobin is generally termed while in the 
veins, deoxy- or reduced hemoglobin. 

The opacity of the blood or the inability to see objects through it, is 
the result of the dissipation of light, caused by the shape of the red 
corpuscles. 

The specific gravity within the limits of health, ranges from 1.045 to 
1.075 though the average is about 1.056. 

The reaction of the blood is stated to be alkaline though in reality it is 
neutral inasmuch as the hydrogen ions and the hydroxyl ions are present 
in equal amounts. 

The temperature varies in different regions. In the aorta it is approxi- 
mated 38.6'C; in the portal vein 39°C; in the hepatic veins 3Q.7°C. 

The viscosity or the resistance to the movement of the molecules of 
the plasma among themselves, together with that of the corpuscles, is 
considerable. Compared with distilled water the viscosity of human 
blood is 4.5 times as great. The viscosity is increased and decreased by a 



96 HUMAN PHYSIOLOGY 

rise or fall in the number of red corpuscles. In a case of polycythemia, 
the red corpuscle count was 11,000,000 per cubic millimeter and the vis- 
cosity 3 and 4 times the normal. 

Coagulability. — When blood is withdrawn from the body and allowed 
to remain at rest, it becomes somewhat thick and viscid in from three to 
five minutes; this viscidity gradually increases until the entire volume of 
blood assumes a jelly-like consistence, which process occupies from five 
to fifteen minutes. 

If a portion of such a jelly-like mass be examined microscopically, it 
will be found to be penetrated in all directions by a felt- work of extremely 
fine delicate fibrils, which, having made their appearance before the 
corpuscles have had time to settle to the bottom of the fluid, have en- 
tangled them in the meshes so that the entire mass retains its characteristic 
color. These fibrils are collectively known as fibrin. The appearance of 
the fibrin is, therefore, the cause of the coagulation. 

As soon as coagulation is completed, a second process begins, which 
consists in the contraction of the coagulum and the oozing of a clear, 
straw-colored liquid — the serum — which gradually increases in quantity 
as the clot diminishes in size, by contraction, until the separation is 
completed, which occupies from twelve to twenty-four hours. 

The Cause of Coagulation. — Coagulation is due to the appearance of 
fibrin, a compound formed by a physico-chemic union of an organic 
colloidal body, thrombin, with fibrinogen, this latter substance being 
always present in the blood. Thrombin is believed to be a derivative 
of an antecedent substance prothrombin or thrombogen, a substance 
always present in the blood and is a product of the decomposition of 
leukocytes and the blood-platelets. With thrombin there is associated a 
calcium salt which is essential for coagulation. If it is removed by the 
addition of potassium oxalate coagulation does not take place. These 
three substances prothrombin or thrombogen, a calcium salt and fibrino- 
gen are always present in the blood. The formation of thrombin which 
would cause coagulation is prevented by the presence of an anti-thrombin. 
As soon as blood is shed or tissues are injured a new substance throm- 
binoplastin is developed which neutralizes the anti-thrombin. This hav- 
ing been accomplished the calcium is enabled to activate the prothrombin 
with the production of thrombin and hence fibrin (Howell.) 

Conditions Influencing Coagulation. — The process is retarded by cold, 
retention within living normal vessels, neutral salts in excess, the injection 
of commercial peptone, etc. 



THE BLOOD 97 

It is accelerated by a temperature of ioo°F., contact with rough surfaces, 
the presence of foreign bodies, whipping, etc. 

Blood coagulates in the body after the arrest of the circulation in the 
course of twelve to twenty-four hours; local arrest of the circulation, from 
compression or a ligature with injury to the lining membrane of the vessel, 
will cause coagulation, thus preventing hemorrhages from wounded 
vessels. 

Chemic Composition of Plasma. — An average composition of plasma 
is shown in the following table: 

Water 90 . 00 

S Plasma-albumin 4-50 

Paraglobulin 3.40 

[ Fibrinogen o . 30 

Fatty matters 0.25 

Sugar 0.10 

Extractives o . 60 

Inorganic salts 0.85 



The water imparts fluidity to the blood and acts as a solvent for the 
inorganic matters, for sugar, and various products of katabolism. Plasma- 
albumin was formerly regarded as the nutritive protein of the blood and 
directly used as such by the tissue elements. As the amino-acids are now 
believed to play this role, the albumin must have some other function. 
It may be a reserve supply of protein food. 

Paraglobulin is a soft, amorphous substance precipitated by sodium 
chloride in excess, or by passing a stream of carbonic acid through dilute 
serum. 

Fibrinogen also can be obtained by strongly diluting the serum and pass- 
ing carbonic acid through it for a long time, when it is precipitated as a 
viscous deposit. 

Fatty matter exists in the blood to the extent of about 0.25 per cent. Just 
after a meal rich in fat, this amount may be considerably increased. 
Within a few hours it disappears, though its ultimate fate is unknown. 

Sugar is represented by dextrose. The amount present varies from 0.1 
to 0.2 per cent. It is derived directly from the glycogen of the liver. 
Should the normal percentage be increased, the sugar is eliminated by 
the kidn. 

The inorganic constituents are chiefly sodium and potassium chlorids, 
sulphates and phosphates together with calcium and magnesium phos- 
7 



98 HUMAN PHYSIOLOGY 

phates. The sodium chlorid is the most abundant, amounting to about 
5.5 parts per thousand. The alkaline salts impart the alkaline reaction 
and promote the absorption from the tissues of the carbon dioxid. 

Excrementitious matters are represented by carbonic acid, urea, creatin, 
creatinin, urates, oxalates, etc.; they are absorbed from the tissues by the 
blood and conveyed to the excretory organs, lungs, kidneys, etc. 

Gases. — Oxygen, nitrogen, and carbonic acid exist in varying propor- 
tions. 

The serum differs from plasma in not containing those materials which 
entered into the formation of fibrin. 

THE RED CORPUSCLES OR ERYTHROCYTES 

The red corpuscles are circular biconcave flattened discs having an 
average diameter of 0.007 mm. or about 3^,200 of an inch. A single cor- 
puscle is of a pale straw color. It is only when aggregated in masses 
that they assume a red color. In man and mammals the red corpuscles 
present neither a nucleus nor a cell wall and are universally of a small size, 
though the size varies considerable in different mammals. 

The red corpuscles are exceedingly numerous, amounting to about 
5,000,000 in a cubic millimeter of blood. In structure they consist of a 
firm, elastic, colorless framework — the stroma — in the meshes of which is 
entangled the coloring-matter — the hemoglobin. 

According to some histologists the red corpuscle, while in the plasma, 
assumes a bell shape. The circular biconcave shape usually observed 
under the microscope is regarded as due to cooling and evaporation and 
concentration of the drawn blood. 

In the birds, reptiles, amphibia, and fish the red corpuscles are oval in 
shape and have a distinct nucleus. They can, therefore, be readily dis- 
inguished from the corpuscles of mammals, not only by their structure 
but also by their size, which is distinctly larger. 

Chemic Composition of Red Corpuscles. — When analyzed chemically 
the red corpuscles are found to consist of water 65 per cent, and solid 
matter 35 per cent. The solids, moreover, have been found to consist of 
a pigment hemoglobin 33, protein 0.9, cholesterin and lecithin 0.46, and 
inorganic salts (chiefly potassium phosphate and chlorid and sodium 
chlorid) 1.4 per cent, respectively. Of the total solids the hemoglobin 
constitutes about 94 per cent. 

Hemoglobin, the coloring-matter of the corpuscles, is an albuminous 
compound, composed of C,0,H,N,S, and iron. It may exist in either 
an amorphous or a crystalline form. When deprived of all its oxygen, 



THE BLOOD 99 

except the quantity entering into its intimate composition, the hemoglobin 
becomes purplish in color, and is known as reduced hemoglobin. When ex- 
posed to the action of oxygen, it again absorbs a definite amount and be- 
comes scarlet in color, and is known as oxyhemoglobin. The amount of 
oxygen absorbed is 1.34 c.c. for each gram of hemoglobin. 

It is this substance which gives the color to the venous and arterial 
blood. As the venous blood passes through the capillaries of the lungs 
the reduced hemoglobin absorbs the oxygen from the pulmonary air and 
becomes oxyhemoglobin, scarlet in color; the blood becomes arterial. 
When the arterial blood passes into the systemic capillaries, the oxygen 
is absorbed by the tissues; the hemoglobin becomes reduced, purple in 
color, and the blood becomes venous. A dilute solution of oxyhemoglobin 
gives two absorption bands between the lines D and E of the solar spec- 
trum. Reduced hemoglobin gives but one absorption band, occupying 
the space existing between the two bands of the oxyhemoglobin spectrum. 

The Function of the Red Corpuscles. — The red corpuscles, by virtue 
of the capacity of their contained hemoglobin for oxygen absorption, may 
be regarded as carriers of oxygen from the lungs to the tissues, and there- 
fore important factors in the general respiratory process. The size as well 
as the number of the corpuscles in different classes of animals appears to 
be directly related to the activity of the respiratory process. In those 
animals in which the corpuscles are small and numerous and the total 
superficial area large, respiration is active, the quantity of oxygen absorbed 
is large, and the energy liberated through oxidation is correspondingly 
large. In those animals, on the contrary, in which the corpuscles are 
large and relatively few in number, the reverse conditions obtain. This 
is in accordance with the fact that the superficial area of any given volume 
of substance is increased in proportion to the extent to which it is sub- 
divided. 

Origin. — The red corpuscles are derived from erythroblasts found in 
the red marrow of the long bones. In the passages of the capillary net- 
work of the marrow, the erythroblasts make their appearance most 
probably by a transformation of pre-existing marrow cells which cross the 
capillary wall from without. At first they are large, homogeneous, color- 
less, perhaps slightly tinged with hemoglobin and distinctly nucleated. 
They increase in number by karyokinesis and at the same time increase in 
their hemoglobin content. In the course of their development the nucleus 
becomes smaller and denser, when the cells are known as normoblasts. 
Subsequently the nucleus is extruded, carrying with it a portion of the 
perinuclear cytoplasm, after which the remainder of the corpuscle assumes 



IOO HUMAN PHYSIOLOGY 

the shape and size of the adult corpuscle and is carried out into the general 
circulation. After severe hemorrhage the formative processes in the 
marrow may become so active that erythroblasts and normoblasts make 
their appearance in the blood-stream before the extrusion of the nucleus 
has taken place. 

THE WHITE CORPUSCLES OR LEUKOCYTES 

The white corpuscle is grayish in color, round or globular in form 
though often presenting a more or less irregular surface. Its diameter is 
about o.on mm. or about M.500 of an inch. Some of the white cor- 
puscles are, however, somewhat larger and others smaller. 

A typical white corpuscle consists of a ground substance uniformly 
transparent and apparently homogeneous in which are embedded a num- 
ber of granules of varying size, some of which are very fine, while others 
are large. By various reagents it has been demonstrated that the gran- 
ules are fatty, protein, and carbohydrate (glycogen) in character. In 
the fresh cells the existence of a nucleus is difficult of detection, though its 
presence can be demonstrated by the addition of acetic acid, which renders 
the perinuclear cytoplasm more transparent and makes the nucleus con* 
spicuous and sharply defined. 

The number of white corpuscles per cubic millimeter of blood is much 
less than the number of red corpuscles, the ratio being in the neighborhood 
of i white to 700 red. This ratio, however, varies within wide limits in 
different portions of the body and under normal variations in physiologic 
conditions. In the blood of the splenic artery there is but 1 white to 2,260 
red, while in the splenic vein there is 1 white to every 60 red; or about 
thirty-eight times as many as in the artery. In the portal vein there is 1 
white to 740 red, while in the hepatic vein there is 1 white to 170 red. 

The total number of white corpuscles per cubic millimeter has been 
estimated at from 5,000 to 10,000, though the average is about 7,500. The 
number, however, is influenced by a variety of physiologic conditions. 

The white corpuscles are usually divided into 

T , / Small 25 per cent. 

1. Lymphocytes < T „ 

r 1 Large 4 to 8 per cent. 

T . j Polymorphonuclear 60 to 70 per cent. 

2. Leukocytes < _ . . ., 

{ Eosinophils 0.5 to 2 per cent. 

In abnormal conditions of the blood other forms of leukocytes are fre- 
quently present, e. g., myelocytes, leukoblasts, myeloplaxes, etc., the 
significance of which is not always apparent. 



THE BLOOD IOI 

Properties. — The white corpuscles possess the power of spontaneous 
movement, alternately contracting and expanding, throwing out processes 
of their substance and quickly withdrawing them, thus changing their 
shape from moment to moment. These movements resemble those of 
the ameba, and for this reason are termed ameboid. The white corpuscles 
also possess the capability of passing through the walls of the capillaries 
into the surrounding tissue spaces; to this process the term diapedesis is 
given. 

Functions. — The functions of the white corpuscles are but imperfectly 
known, and at present no positive statements can be made. It has been 
suggested that wherever found in the body, whether in blood or tissues, 
they are engaged in the removal of more or less insoluble particles of dis- 
integrated tissues, in attacking and destroying more or less effectively 
various forms of invading bacteria and thus protecting the body against 
their deleterious activity. This they do by surrounding, enveloping, and 
incorporating either the tissue particle or bacterium and digesting it. 
On account of this swallowing action these cells were termed by Metchni- 
korl phagocytes and the process phagocytosis. The cells engaged in this 
process are the polymorphonuclear leukocytes and the large and the small 
lymphocytes. He regards them as the general scavengers of the body. 
It has been suggested that they are also engaged in the absorption of fat 
from the lymphoid tissue of the intestine. In their dissolution they con- 
tribute to the blood-plasma certain protein materials which assist under 
favorable circumstances in the coagulation of the blood. 

Origin. — The first group of the white corpuscles — lymphocytes — take 
their origin entirely from the lymph-adenoid tissues of the body, e.g., the 
lymph-glands, solitary and agminated follicles of the intestines, etc. As 
the lymph flows through these structures the lymph-corpuscles, as the 
future lymphocytes of the blood are called in these situations, are washed 
out and carried by way of the lymph-stream into the general circulation. 

The second group — the polymorphonuclear, the eosinophiles and baso- 
phil leukocytes have their origin in the bone marrow. The immediate 
ancestors of these cells are known as myelocytes and are normally found 
in the red bone-marrow. These cells, through transitional stages, assume 
the characteristics of the leukocytes just mentioned and pass directly 
into the capillaries of the marrow whence they are distributed throughout 
the body. 

After an unknown period of life the leukocytes undergo dissolution and 
disappear. 



102 HUMAN PHYSIOLOGY 

Blood Platelets. — These are small histologic elements circulating in 
the blood though their presence can not be readily determined except 
under special conditions. They are colorless homogeneous or finely 
granular non-nucleated disks which vary in diameter from 1.5 to 3.5 
micro-millimeters. They number from 250,000 to 300,000 per cubic 
millimeter of blood. They are supposed to represent fragments of the 
cystoplasm of giant cells found in the marrow of bones. They are be- 
lieved to be connected in some way with the coagulation of the blood. 

THE CIRCULATION OF THE BLOOD 

The circulatory apparatus by which the blood is distributed to and re- 
moved from all regions of the body consists of a central organ, the heart; 
a series of branching diverging tubes, the arteries; a network of minute 
passageways with extremely delicate walls, the capillaries; a series of con- 
verging tubes, the veins. These structures are so arranged as to form a 
closed system of vessels within which the blood is kept in continuous move- 
ment mainly by the pressure produced by the pumping action of the heart, 
though aided by other forces. By reason of its general arrangement 
and activity the tissues are continuously supplied with nutritive materials 
and freed from their waste materials and carried to the eliminating organs. 

The Heart. — The heart is a conic or pyramid-shaped hollow muscle 
situated in the thorax just behind the sternum. The base is directed up- 
ward and to the right side; the apex downward and to the left side, extend- 
ing as far as the space between the cartilages of the fifth and sixth ribs. 
In this situation the heart is enclosed and suspended in a fibro-serous sac, 
the pericardium, attached to the great vessels at its base. 

Cavities of the Heart. — The general cavity of the heart is subdivided 
by a longitudinal septum into a right and left half; each of these cavities is 
in turn subdivided by a transverse septum into two smaller cavities, which 
communicate with each other and are known as the auricles and ventricles; 
the orifice between the auricle and ventricle being known as the auriculo- 
ventricular orifice. The heart, therefore, consists of four cavities — a right 
auricle and ventricle and a left auricle and ventricle. 

The right auricle and the right ventricle constitute the venous heart; 
the left auricle and left ventricle constitute the arterial heart. 

Into the right auricle the two terminal trunks of the venous system — the 
superior [and inferior vena cava — empty the venous blood which has been 
collected from all parts of the system; from the right ventricle arises the 
pulmonic artery, which, passing into the lungs, distributes the blood to the 



CIRCULATION OF THE BLOOD 



I°3 



walls of the air-cells of the lungs; into 
the left auricle empty four pulmonic 
veins, which have collected the blood 
from the lung capillaries; from the left 
ventricle springs the aorta, the general 
trunk of the arterial system, the branches 
of which distribute the blood to the 
entire system. 

The Course of the Blood through the 
Heart. — Reference to Fig. n will make 
it clear that there is a pathway for the 
blood between the venae cavae on the 
right side and the aorta on the left side 
by way of the right side of the heart, 
the cardio-pulmonic vessels and the left 
side of the heart. 

The venous blood flowing toward the 
heart is emptied by the superior and 
inferior vena cava into the right auricle 
from which it passes through the auric- 
ulo ventricular opening into the right 
ventricle; thence into and through the 
pulmonic artery and its branches to the 
pulmonic capillaries where it is arterial- 
ized, i.e., yields up a portion of its car- 
bon dioxid and takes on a fresh supply 
of oxygen — and is changed in color from 
bluish red to scarlet red. The arterial- 
ized blood flowing toward the heart is 
emptied by the pulmonic veins into the 
left auricle from which it passes through 
the auriculoventricular opening into 
the left ventricle; thence into the aorta 
and its branches to the systemic capil- 
laries where it is dcarterialized by an 
opposite exchange of gases, i.e., yields 
up a portion of its oxygen to, and al>- 



Fig. ii. — Diagram of the Circulation. 
i i. Heart. 2. Lungs. 3. Head and 
upper extremities. 4. Spleen. 5. Intestines. 
6. Kidney. 7. Lower extremities. 8. Liver. 
—{After Dalton.) 




104 HUMAN PHYSIOLOGY 

sorbs carbon dioxid from the tissues, and changes in color, from scarlet 
red to bluish red. The venous blood is again returned to the systemic 
veins to the venae cavae. 

Though the blood is thus described as flowing first through the right 
side and then through the left side, it must be kept in mind that the two 
sides fill synchronously; that while the blood is flowing into the right side 
from the venae cavae, it is also flowing into the left side from the pulmonic 
veins in equal quantities and velocities. 

While there is but one circulation, physiologists frequently divide the 
circulatory apparatus into: 

i. The systemic circulation, which includes the movement of the blood 
from the left side of the heart through the aorta and its branches, through 
the capillaries and veins, to the right side of the heart. 

2. The pulmonic circulation, which includes the course of the blood 
from the right side through the pulmonic artery, through the capillaries 
of the lungs and pulmonic veins, to the left side of the heart. 

3. The portal circulation, which includes the portal vein. This vein is 
formed by the union of the radicles of the gastric, mesenteric, and splenic 
veins, and carries the blood directly into the liver, where the vein divides 
into a fine capillary plexus, from which the hepatic veins arise; these 
empty into the ascending vena cava. 

Orifices and Valves. — The movement of the blood along the path of 
the circle above outlined is accomplished by the alternate contraction and 
relaxation of the muscle walls of the heart. That the movement may be a 
progressive one, that there shall be no regurgitation during either the 
contraction or the relaxation, it is essential that some of the orifices of 
the heart be closed during each of these periods. This is accomplished 
by the heart valves. 

The valves of the heart are formed by a reduplication of the endocardium 
strengthened by connective tissue. 

The right auriculo-ventricular opening is provided with a valve con- 
sisting of three portions or cusps which during the period of relaxation 
are directed into the ventricle; during the contraction they are raised and 
placed in complete apposition when they act as a valve preventing a 
backward flow into the auricle. For this reason it is known as the tri- 
cuspid valve. The left auriculo-ventricular orifice is provided with a valve 
consisting of but two cusps and is, therefore, termed the bicuspid valve, 
or, from its fancied resemblance to a bishop's miter, the mitral valve. 
The mode of action of this valve is similar in all respects to the tricuspid 
valve. To the undersurface and to the edges of these valves the tendinous 



CIRCULATION OF THE BLOOD 105 

cords of the papillary muscles are firmly and intricately attached. These 
cords are just sufficiently long to permit closure of the valves and to 
prevent them from being floated into the auricle. 

The orifice of the pulmonic artery is provided with three semilunar or 
pocket-shaped membranes, the semilunar valves. The orifice of the 
aorta is also provided with three similarly arranged semilunar membranes, 
the semilunar vahes. 

During the period of relaxation of the heart the edges of the semilunar 
valves are in close apposition and prevent a return of the blood into the 
ventricles; during the contraction they are directed into the pulmonic 
artery and aorta. In the former position they are shut; in the latter, 
the}' are open. 

The Auriculo -ventricular Bundle. — This is a specialized bundle of 
muscle-fibers discovered in part by His and in part by Tawara which 
unites anatomically and physiologically the right auricle with the ventricles. 
The reason for the existence of this bundle lies in the fact that the muscle 
fibers of the auricles and ventricles are completely separated by the 
transverse fibrous septum to which they are attached. The origin, course 
and distribution of this bundle is as follows: 

It arises near the opening of the coronary sinus where it is connected 
with the true auricular fibers. From their origin the fibers converge to 
form a distinct bundle which then passes forward on the right side of the 
auricular septum between the lower edge of the fossa ovalis and the 
auriculo-ventricular septum; just above the insertion of the median cusp 
of the tricuspid valve the bundle presents a very complicated network 
of muscle-fibers which has been designated as a knot or the auriculo- 
ventricular node or the node of Tawara; from the anterior portion of the 
node a bundle of fibers turns downward and penetrates the auriculo- 
ventricular septum, beyond which it passes below the pars membranacca 
scpti to the upper limit of the muscle portion of the ventricular septum. 
It then divides into two limbs or branches which descend on either side 
of the septum under the endocardium, the right limb lying somewhat 
deeper than the left. Each of these limbs is enclosed by a layer of con- 
nective tissue which isolates it from the musculature of the ventricular 
septum as far as the lower third of the ventricular cavities. In this 
region they divide into a number of bundles, some of which enter the 
papillary muscles, while others, forming tendon-like strands, branch 
freely beneath the endocardium and spread in all directions over the 
entire inner surface of the ventricle and enter into histologic connection 
with the true cardiac mus< Ic-fibers. The ultimate terminations of this 
system beneath the endocardium constitutes the so-called Purkinje- 



106 HUMAN PHYSIOLOGY 

fiber layer. From its function this bundle has been termed the conduc- 
tion system of the heart — the conduction of an excitation process from the 
auricle to the ventricles. 

The Sino-auricular Node or the Keith-Flack Node. — This is a small mass 
of primitive muscle tissue situated in the sulcus terminalis at the junction 
of the superior vena cava and the auricular appendix. It is supplied 
with blood-vessels and nerves. From the node muscle-fibers pass along 
the sulcus for a distance of two centimeters and finally becomes connected 
with the true auricular fibers. Experimental investigations lead to the 
inference that it is directly concerned in the initiation of the heart beat. 
It has been designated the "pace-maker" of the heart. 

The Mechanics of the Heart. — With each beat, the heart presents two 
distinct movements which alternate with each other in quick succession. 
One is the movement of contraction, or the systole, by which the blood 
contained within its cavities is ejected into the arteries — pulmonic artery 
and aorta; the other is the movement of relaxation, or the diastole, fol- 
lowed by a pause during which the cavities again fill up with blood from 
the venae cavae and pulmonic veins. 

The contraction of any part of the heart is termed the systole; the re- 
laxation, the diastole. As each side of the heart has two cavities the 
walls of which contract and relax in succession, it is customary to speak 
of an auricular systole and diastole, and a ventricular systole and diastole. 
As the two sides of the heart are in the same anatomic relation to each 
other, they contract and relax in the same periods of time. 

The immediate cause of the movement of the blood through the vessels 
is the contraction and relaxation of the muscle- walls of the heart, and more 
particularly of the walls of the ventricles, each of which plays alternately 
the part of a force-pump, and possibly to a slight extent of a suction-pump. 
The motive power is furnished by the heart itself, by the transformation 
of potential energy, stored up during the period of rest, into kinetic energy 
— i.e., heat and mechanic motion. 

The Cardiac Impulse. — In passing from the diastolic to the systolic 
condition the transverse diameter diminishes while the antero-posterior 
diameter increases, and the whole heart becomes somewhat more conic 
in shape. It is questionable if the vertical diameter perceptibly shortens. 
During the systole the heart hardens, increases in convexity, and is more 
forcibly pressed against the chest wall. As this takes place suddenly, 
it gives rise to a marked vibration of the chest wall, known as the cardiac 
impulse. 



CIRCULATION OF THE BLOOD 107 

This impulse is principally observed in the space between the fifth and 
sixth ribs about an inch internal to a line drawn vertically from the 
middle of the clavicle. The cardiac impulse is synchronous with the 
cardiac systole. 

The Cardiac Cycle. — The term cardiac cycle is employed to express the 
sequence of events from the beginning of one auricular systole and the 
beginning of the auricular systole which immediately follows it. An 
examination of the heart shows that each pulsation may be divided into 
three phases, viz.: 

1. The auricular systole. 

2. The ventricular systole. 

3. The pause or period of repose during which both auricles and 
ventricles are at rest. 

The duration of a cycle, as well as the duration of its three stages, varies 
in different animals in accordance with the number of cycles which recur 
in a minute. In human beings in adult life there are about 72 cycles to 
the minute; the average duration is, therefore, 0.80 sec. From this it 
follows that the time occupied by any one of the three stages must be 
extremely short and difficult of determination. From experiments on 
animals and from observations made on human beings, the following 
estimates have been made and accepted as approximately correct for 
human beings: 

1. The auricular systole — 0.16 sec; the auricular diastole, 0.64 sec. 

2. The ventricular systole — 0.32 sec; the ventricular diastole, 0.48 sec. 

3. The period of rest for both auricles and ventricles — 0.32 sec 

The Movement of the Blood during the Cycle. — It is apparent that 
with the relaxation of the auricular walls blood at once flows from the 
venae cavae and the pulmonic veins into the auricular cavities and continues 
so to do throughout the entire auricular diastole. With the relaxation of 
the ventricular walls, however, \he blood that has accumulated in the 
auricles up to this time, or its equivalent coming from the venae cavae 
and pulmonic veins, now flows into the ventricles until they are nearly 
filled. Before they are filled, however, the auricular diastole comes to 
an end, the auricular walls again contract and force some of their con- 
tained blood into the ventricles and thus rapidly complete the filling. 
The ventricular systole immediately follows, during which the blood is 
driven into the pulmonic artery and aorta. This having Jbeen accom- 
plished, the ventricles relax, and the blood that has been accumulating 
in the auricles begins to flow into the ventricles, after which the same 
series of events follows as in the previous cycle. 



108 HUMAN PHYSIOLOGY 

The Action of the Valves. — The forward movement of the blood is 
permitted and regurgitation prevented by the alternate action of the 
auriculo- ventricular and semilunar valves. As a point of departure for 
a consideration of the action of these valves and their relation to the 
systole and diastole of the heart, the close of the ventricular systole 
may be selected. At this moment, the semilunar valves, which during 
the systole, are directed outward by the blood current are now suddenly 
and completely closed by the pressure of the blood in the aorta and 
pulmonic artery. Regurgitation into the ventricles is thus prevented. 

During the ventricular systole, the relaxed auricles are filling with 
blood. With the ventricular diastole this blood or its equivalent flows 
into the relaxed and easily distensible ventricles until both auricles and 
ventricles are nearly filled. The tricuspid and mitral valves which are 
hanging down into the ventricular cavities, are now floated up by currents 
of blood welling up behind them until they are nearly closed. The 
auricles now suddenly contract, forcing their contained volumes into the 
ventricles which become fully distended. 

With the cessation of the auricular systole, the ventricular systole 
begins. If the blood is not to be returned to the auricles the tricuspid 
and mitral valves must be instantly and completely closed. This is 
accomplished by the upward pressure of the blood which brings their 
free edges in close apposition. Reversal in the position of these valves 
is prevented by the contraction of the papillary muscles which exert a 
traction on their undersurfaces and edges and hold them steady. 

The blood now confined in the ventricles between the closed auriculo- 
ventricular and semilunar valves is subjected to pressure on all sides; as 
the pressure rises proportionately to the vigor of the contraction there 
comes a moment when the infra-ventricular pressure exceeds that in the 
aorta and pulmonic artery; at once the semilunar valves are thrown open 
and the blood discharged. Both contraction and outflow continue until 
the ventricles are practically empty, when relaxation sets in attended 
by a rapid fall of pressure. Under the influence of the positive pressure of 
the blood in the aorta and pulmonic artery, the semilunar valves are 
again closed. The accumulation of blood in the auricles, attended by a 
rise in pressure, again forces the tricuspid and mitral valves open. With 
these events the cardiac cycle is completed. 

Sounds of the Heart. — If the ear be placed over the cardiac region, 
two distinct sounds are heard during each revolution of the heart, closely 
following each other, and which differ in character. 

The sound coinciding with the systole in point of time — the first sound — 
is prolonged and dull, and caused by the closure and vibration of the 



CIRCULATION OP THE BLOOD 109 

auriculo-ventricular valves, the contraction of the walls of the ventricles, 
and the apex-beat; the second sound, occurring at the beginning of the 
diastole, with the second phase of the cardiac cycle is short and sharp, 
and caused by the closure and vibration of the semilunar valves. 

The Intra -ventricular Pressures. — By this term is meant the pressure 
that arises in the ventricles during the time of the systole. The reason 
for this rise of pressure arises from the fact that the semilunar valves 
are kept tightly shut by the pressure of the blood in the aorta and pul- 
monic artery. With the beginning of the systole the auriculo-ventricular 
valves are suddenly closed and now the blood is imprisoned. If the semi- 
lunar valves are to be opened and the blood discharged the intra-ventricular 
pressure must exceed the pressure in the aorta and pulmonic artery. 
Moreover as the aortic and pulmonic pressures increase with the dis- 
charge of blood the intra-ventricular pressure must continue to rise and 
exceed the increased pressure in these vessels. This the heart does by 
calling on the reserve power with which it is endowed. Should it fail to 
meet some sudden rise of pressure in the aorta it would remain in a 
condition of permanent diastole. 

The Frequency of the Heart-beat. — The frequency of the heart-beat 
varies with a variety of conditions: e.g., age, sex, posture, exercise, etc. 

Age. — The most important normal condition which modifies the activity 
of the heart is age. Thus: 

Before birth, the number of beats a minute averages 140 

During the first year it diminishes to 128 

During the third year it diminishes to 95 

From the eighth to the fourteenth year it averages 84 

In adult males it averages 72 

Sex. — The heart-beat is more rapid in females than in males. Thus 
while the average beat in males is 72, in females it is usually 8 or 10 beats 
more. 

Posture. — Independent of muscle efforts the rate of the beat is influenced 
by posture. It has been found that when the body is changed from the 
lying to the sitting and to the standing position, the beat will vary as 
follows — from 66 to 71 to 81 on the average. 

rcise and digestion also temporarily increase the number of beats. 

A rise in blood-pressure from any cause whatever is usually attended by 
a de hile a Jail in blood-pressure is attended by an increase in the 

rate. 



TTO HUMAN PHYSIOLOGY 

The Blood Supply to the Heart. — The nutrition of the heart, its con- 
tractility, the force and frequency of the beat are dependent on and 
maintained by the introduction of arterialized blood into and the removal 
of waste products from its tissue. This is accomplished by the coronary 
arteries and the coronary veins. The arteries and veins on the surface 
of the heart are known as the extra-mural arteries and veins; those which 
are found in the substance of the heart are known as intra-mural arteries 
and veins. During the time of the systole the extra-mural arteries are 
filled with blood from the aorta; during the time of the diastole, the blood 
flows into the intra-mural arteries and capillaries, furnishing to the muscle 
cells an additional supply of nutritive materials and receiving products of 
waste; at the succeeding systole the venous blood is driven from the 
intra-mural into the extra-mural veins and so into the right auricle. 

The Causation of the Heart-beat. — From the fact that the heart will 
continue to beat for a variable length of time after removal from the 
body (the time varying with the species of animal from which it has been 
obtained) it is evident that the beat is independent of the central nerve 
system. * 

The fundamental condition for the continuance of the beat is the main- 
tenance of the irritability. So long as this persists the heart will respond 
to its appropriate stimulus. The irritability of the heart within the body 
is dependent on the supply of blood coming through its nutrient vessels 
or flowing through its cavities. Outside the body, the irritability can 
be maintained for some hours by perfusing the coronary system of vessels 
with the Ringer-Locke solution. 

The Nature of the Stimulus. — The presence of nerve-cells in the walls of 
the heart, their relation to the muscle cells, the pronounced activity of the 
sinus of the frog heart where they are very abundant; the feeble activity 
of the apex where they are absent gave rise to the idea that the stimulus is 
a nerve impulse rhythmically and automatically discharged by these nerve- 
cells. This view is no longer entertained. It has been demonstrated that 
portions of the heart muscle, that do not contain nerve-cells, will continue 
to exhibit rhythmic contraction for some hours if supplied with oxygenated 
and defibrinated blood; that the embryonic heart contracts rhythmically 
before nerve-cells have migrated to its walls. 

The stimulus therefore evidently arises within the heart muscle. In 
other words, it is myogenic and not neurogenic. The stimulus is now be- 
lieved to be chemic in character and due to a reaction between the 
inorganic salts in the muscle cells and those in lymph by which they are 
surrounded. 



CIRCULATION OF THE BLOOD III 

The Influence of the Central Nerve System on the Action of the 
Heart — Though the heart-beat is independent of the central nerve system, 
it is to a considerable extent modified by it either in the way of acceleration 
or inhibition. In all classes of animals the heart not only contains localized 
collections of nerve-cells, but it is also connected with the central nerve 
system by two sets of nerve-fibers. 

In the frog heart a group of nerve-cells is found in the sinus at its junction 
with the auricle, and known as the crescent or ganglion of Remak; a second 
group is found at the base of the ventricle on its anterior aspect and known 
as the ganglion of Bidder; a third group is found in the auricular septum, 
known as the septal ganglion, or the ganglion of Ludwig. 

In the dog and the mammalian heart generally, the nerve-cells though 
present are not arranged in such definite groups, but are distributed in the 
terminations of the venae cavae, pulmonic veins, the walls of the auricles 
and in the neighborhood of the base of the ventricles. 

These cells were formerly regarded as the source of the stimuli for the 
excitation and regulation of the heart's contraction. This view is no 
longer entertained. 

The extra-cardiac nerves, those which connect the heart with the 
central nerve system, are the sympathetic and the vagus. Experiments 
have demonstrated that the sympathetic is the motor nerve to the heart, 
the nerve that accelerates the rate and augments the force of the beat, 
while the vagus is the inhibitor nerve, the nerve that inhibits or controls 
the rate and force of the beat. 

Since the heart muscle belongs to the autonomic tissues, it follows that 
the accelerator and the inhibitor nerve pathways consist of two consecu- 
tively arranged neurons. The first is termed preganglionic, the second 
postganglionic. 

The Sympathetic. — The preganglionic fibers have their origin in the 
medulla oblongata and very probably from nerve-cells in the gray matter 
beneath the floor of the fourth ventricle. From this origin they descend 
the spinal cord as far as the level of the second, third, and at times the 
fourth thoracic nerves. At this level they emerge from the cord in com- 
pany with the nerve-fibers composing the anterior roots of the second, 
third, and fourth thoracic nerves. After a short course, they enter the 
white rami communicantes, then the sympathetic chain and pass upward 
to the ganglion stellatum (the first thoracic), and to the inferior cervical 
I ganglion as well, around the nerve-cells of both of which their terminal 
, branches arborize. From the nerve-cells of both the stellate and inferior 
cervical ganglia, the postganglionic fibers arise, that is, the sympathetic 
nerves proper, which after emerging from the ganglia pass toward the 



112 HUMAN PHYSIOLOGY 

heart. On reaching the heart they terminate directly in the muscle- 
cell, or indirectly through the intermediation of intra-cardiac nerve-cells. 
The former mode of termination is the more probable. 

Stimulation of these fibers in any part of their course, more readily the 
sympathetic fibers after their emergence from the ganglia, is followed by 
an increase in the rate and sometimes by an increase in the force of the 
heart-beat. For this reason the sympathetic is said to exert an accelerator 
and an augmentor influence on the heart-beat. 

The percentage increase in the acceleration varies in different animals. 
In some instances the increase varies from 58 per cent, to 100 per cent. 
If the heart is beating slowly before stimulation, the acceleration is more 
marked than if it is beating rapidly. 

Division of the sympathetic nerves is at once followed by a diminution 
in the rate, the degree of which will depend to some extent on the rate 
at which the heart was beating prior to the division. The results, there- 
fore, that follow stimulation and division of these nerves indicate that 
they are transmitting nerve impulses from the centers from which they 
arise to the heart, upon which they exert a stimulating influence on the 
rate arid force of the beat. 

The group of cells from which the accelerator fibers arise is known as the 
cardio-accelerator center. It is believed to be in a state of continuous or 
tonic activity. 

The Vagus. — The preganglionic fibers have their origin in a group of 
nerve-cells situated beneath the floor of the fourth ventricle. From this 
origin they pass out in the trunk of the vagus proper. In the neighborhood 
of the inferior laryngeal nerves, branches containing efferent fibers are 
given off which pass to the heart. Their terminal branches arborize 
around the intra-cardiac ganglia. From the cells of the ganglia the post- 
ganglionic fibers arise which terminate directly in some of the heart muscle 
fibers. 

Stimulation of the vagus fibers in any part of their course with induced 
electric currents will cause the heart to come to a standstill almost im- 
mediately in the condition of diastole, and may be so kept for a variable 
period, from fifteen to thirty seconds or more, during which its walls are 
relaxed and its cavities filled with blood. On cessation of the stimulation 
the contractions return and in a very short time the former rate and force 
of the beat are regained. If the electric currents are of feeble strength, 
the heart will come to rest gradually, through a gradual diminution in the 
rate and force of the contraction. During the period of inhibition the 
walls are completely relaxed and the cavities filled with blood. 

Division of one vagus is followed in some mammals, e.g., dog by a marked 



CIRCULATION OF THE BLOOD 113 

increase in the rate of beat and if both vagi are divided the increase may 
amount to from 50 to 75 per cent. The results of stimulation and division 
of the vagus nerves indicate that they are continuously transmitting nerve 
impulses from the centers from which they arise, to the heart muscle, on 
the activity of which they exert a restraining or inhibitor influence. 

The center in the medulla from which the inhibitor fibers arise is known 
as the cardio-inhibitor center. This center is also believed to be in a 
state of continuous activity though capable of being increased or de- 
creased in activity by transmitted nerve impulses from various regions of 
the bod)*. 

Reflex acceleration or inhibition of the heart is caused by nerve impulses 
transmitted to the cardio-inhibitor center alone, through afferent nerve- 
fibers, some of which are inhibitor, while others are excitalor. In the 
first instance the center is inhibited in its action whereupon the cardio- 
accelerator center has a freer action and the heart rate is accordingly 
accelerated; in the second instance the center is excited to increased 
activity and its inhibitor effect increased. The effects of the cardio- 
accelerator center is thus in part annulled and the heart rate is diminished. 

THE VASCULAR APPARATUS 

The vascular apparatus in its entirely consists of a closed system oi 
vessels which not only contain the blood, but under the driving power of 
the heart, transmit it to and from all regions of the body. It is usually 
divided into a systemic and a pulmonic portion. 

The Systemic Vascular Apparatus. — This portion of the general vas- 
cular apparatus includes all the vessels extending from the left ventricle to 
the right auricle: viz., the arteries, capillaries, and veins. Though serving 
as a whole to transmit blood from the one side of the heart to the other, 
each one of these three divisions has separate but related functions, which 
are dependent partly on differences in structure and physiologic proper- 
ties, and partly on their relation to the heart and its physiologic activities. 

The Arteries. — The arteries serve to transmit the blood ejected from 
the heart to the capillaries; that this may be accomplished they divide and 
subdivide and ultimately penetrate each and every area of the body. 
Their repeated division is attended by a diminution in size, a decrease in 
the thickness and a change in the structure of their walls. 

A typical artery consists of three coats: an internal, the tunica intima; a 
middle, the tunica media; an external, the tunica adventitia. 

The internal coat consists of a structureless elastic basement membrane, 
the inner surface of which is covered by a layer of elongated spindle-shaped 



114 HUMAN PHYSIOLOGY 

endothelial cells. The middle coat consists of several layers of circularly 
arranged, non-striated muscle-fibers, between which are networks of. 
elastic fibers. The external coat consists of bundles of connective tissue of 
the white fibrous and yellow elastic varieties. Between the external and 
middle coats there is an additional elastic membrane. In the small 
arteries there is but a single layer of muscle-fibers. In the large arteries 
the elastic tissue is very abundant, exceeding largely in amount the muscle- 
tissue. It is also more closely and compactly arranged. The external 
coat is well developed in the large arteries. 

The presence in their walls of both elastic and contractile elements, 
endows the arteries with the two properties of elasticity and contractility. 

Elasticity. — The elasticity is best developed in the large arteries, though 
it is also present in arteries of relatively small size. By virtue of the 
elasticity, the arteries are capable of being distended and elongated and 
when the distending force is removed of recoiling or retracting and return- 
ing to their former condition. The arteries are thus enabled to adapt 
themselves to the variations in the volume of blood discharged from 
the ventricle at a single beat or in a unit of time. The elasticity also con- 
verts the intermittent movement of the blood imparted to it by the heart 
as it is ejected from the ventricle, into a remittent movement in the 
arteries and finally into the continuous and equable movement observed 
in the capillaries. 

Contractility. — The contractility, especially of the small arteries, permits 
of a variation in the amount of blood passing into a given capillary area in 
a unit of time. During the functional activity of any organ or tissue there 
is need for an increase in the amount of blood beyond that supplied during 
functional inactivity or rest. This is accomplished by a relaxation of the 
muscle-fibers. With the cessation of activity the muscle-fibers again 
contract and reduce the amount of blood to that required for nutritive 
purposes only. The tonic contraction of the arteriole muscle-fibers in- 
creases considerably the resistance to the outflow of blood into the capil- 
laries. They thus assist in maintaining the blood-pressure in the arteries. 
This resistance is generally termed the peripheral resistance though 
there is included under this term the resistance offered by the small caliber 
of the capillary blood-vessel as well. This latter factor is constant, the 
former variable. 

The Capillaries. — The capillaries are small vessels continuous with the 
arteries on the one hand and with the veins on the other hand. Though 
different in structure from a small artery or vein, there is no sharp boun- 
dary between them, as their structures pass imperceptibly one into the 



CIRCULATION OF THE BLOOD 115 

other. A true capillary, however, is of uniform size in any given tissue 
and does not undergo any noticeable decrease in size from repeated branch- 
ings. The diameter varies in different tissues from 0.0045 mm - to 0-0075 
mm., just sufficiently large to permit the easy passage of a single red cor- 
puscle. The length varies from 0.5 mm. to 1 mm. The wall of the capil- 
lary is composed of a single layer of nucleated endothelial cells with 
serrated edges united by a cementing material. Though extremely short, 
the capillaries divide and subdivide a number of times, forming meshes or 
networks, the closeness and general arrangement of which vary in different 
localities. 

As the endothelial cells are living structures and characterized by irrita- 
bility, contractility and tonicity, it may be assumed that the capillary wall 
as a whole is characterized by the same properties. Upon the possession 
of these properties the functions of the capillary depend. 

The function of the capillary vessel is to permit of a passage of the nutri- 
tive materials of the blood into the surrounding tissue spaces and of waste 
products from the tissue spaces into the blood. The structure of the capil- 
lary wall is well adapted for this purpose. Composed as it is of but a single 
layer of endothelial cells, the thickness of which defies accurate measure- 
ment, it readily permits, under certain conditions, of the necessary ex- 
change of materials between the blood and the tissues. The forces which 
are concerned in the passage of materials across the capillary wall are em- 
braced under the terms diffusion, osmosis, and filtration. As a result of the 
interchange of materials the tissues are provided with nutritive materials 
and relieved of the presence of the products of metabolism. As the blood 
loses oxygen and gains carbon dioxid, it changes in color from a scarlet red 
to a bluish red. In consequence of the exchange of materials, the blood 
undergoes a change in composition, the extent and character of which 
varies in accordance with the activities and character of the organ tra- 
versed by it. 

In order that the nutritive materials may pass across the capillary 
wall in amounts sufficient to maintain the necessary supply of lymph in the 
lymph or tissue spaces, it is essential that the blood shall flow into and out 
of the capillary vessels constantly and equably, in volumes varying with 
the activities of the tissues, under a given pressure and with a definite 
velocity. The conditions are made possible by the cooperation of the 
physical properties and actions of the heart and vascular apparatus. 

The Veins. — The veins arise from the distal side of the capillary vessels. 
As they emerge they are quite small and designated venules. By their 
convergence and union the veins gradually increase in size in passing from 

\ 



Il6 HUMAN PHYSIOLOGY 

the periphery toward the heart. Their walls at the same time corre- 
spondingly increase in thickness. The veins from the lower extremities, 
the trunk, and abdominal organs finally terminate in the inferior vena 
cava. The veins from the head and upper extremities terminate in the 
superior vena cava. Both venae cavae empty into the right auricle. 

The veins consist of three coats, an internal, a middle and an external 
similar in their composition to most of the arteries. The elastic and 
muscular tissues are, however, not so abundant. 

Veins are distinguished by the possession of valves throughout their 
course, which are arranged in pairs, and formed by a reflection of the in- 
ternal coat, strengthened by fibrous tissues; they always look toward the 
heart, and when closed prevent a reflux of blood in the veins. Valves are 
most numerous in the veins of the extremities, but are entirely absent in 
many others. 

The Flow of the Blood through the Vessels. — During the flow of the 
blood through the arteries, capillaries and veins, certain phenomena are 
presented by each of these three divisions of the vascular apparatus. 
These are mainly velocity and pressure, and in the arteries alone an alter- 
nate expansion and recoil of the arterial wall with each heart-beat, termed 
the pulse. 

Blood -pressure. — Blood-pressure may be defined as the pressure exerted 
radially or laterally by the moving blood stream against the sides of the 
vessels. This pressure is the result of (i) the driving power of the heart, 
and (2) of the resistance offered to the forward movement of the blood — 
a resistance due to the cohesion and friction of the molecules of the blood, 
of the blood corpuscles, and the adhesion of the blood to the sides of the 
blood-vessels. That there is such a pressure within the arteries, capil- 
laries, and veins, different in amount in each of these three divisions of the 
vascular apparatus, is evident from the results which follow division of an 
artery or a vein of corresponding size. When an artery is divided, the 
blood spurts from the opening for a considerable distance and with a cer- 
tain velocity. The reason for this lies in the fact that the vessel has been 
distended by the pressure from within and its walls thrown into a condition 
of elastic tension, so that at the moment there is an outlet, the vessel 
suddenly recoils and forces the blood out with a velocity and to a height 
proportional to the distention. When a vein is divided, the blood as a rule 
merely wells out of the opening with but slight momentum, and for the 
reason that the vessel has been but slightly, if at all distended by the pres- 
sure. These results indicate that the blood in the arteries stands under 
a pressure considerably higher than that of the atmosphere, while that 



CIRCULATION OF THE BLOOD 117 

in the veins stands under a pressure perhaps but slightly above that of the 
atmosphere. Especially true is this of the larger veins. 

Experimentally it has been determined that the pressure in the arteries 
at the end of the cardiac diastole approximates in man about 90 mm. 
Hg: and is termed the diastolic pressure. During the systole and with 
the discharge of blood into the aorta the pressure rises from 30 to 40 mm. 
higher which is then termed the systolic pressure. The pressure in the 
capillaries approximates 20 to 40 mm. and in the veins from 20 to o mm. 
or even less at the terminations of the venae cava?. 

The difference in the height of the pressure in the venous system as 
contrasted with the arterial system is due to the progressive diminution of 
the resistance from the beginning of the aorta to the ends of the venae cavae, 
together with the small diameter of the capillaries, increased to a variable 
extent, by the tonic contraction of the arteriole muscle. 

The Causes of the Blood -pressure. — The Heart. — The primary factor 
in the production of the pressure is the pumping action of the heart. 
Should there be any cessation in its activity, the elastic walls of the ar- 
teries would recoil and force the blood into the veins. There would be 
coincidently a fall of the pressure to that of the atmosphere. Even under 
normal circumstances this condition is approximated during the diastole. 
The recoil of the arterial wall by which the forward movement of the 
blood is maintained is attended by a fall in pressure. But before this 
reaches any considerable extent, the heart again contracts and forces its 
contained volume of blood into the arteries. 

The Resistance. — The secondary factor is the resistance to the flow 
of blood through the vessels, the nature of which has been previously 
stated. So long as the resistance, and especially that variable element of 
it at the periphery of the arterial system, viz., the tonic contraction of the 
arteriole muscle maintains a certain average value, so long will the pres- 
sure in each division of the vascular apparatus maintain an average or a 
physiologic value. Should the resistance at the periphery of the arterial 
m vary in either direction, the result of an increase or a decrease in the 
degree of the contraction of the arteriole muscle, there will arise a change 
in the relative degree of pressure in each of the three divisions of the vas- 
cular apparatus 

The Elasticity of the Vessel Walls. — A tertiary factor is the elasticity of 
the arterial wall. While it can hardly be said that the elasticity is a cause 
of the pressure, there can be attributed to it the capability of modifying 
and 1 in the maintenance of the pressure at a more or less constant 

level; for were it not for this property of the vessel wall the variations in 
pressure during and after the systole would be far more extensive than they 



Il8 HUMAN PHYSIOLOGY 

are, and would approximate the variations observed in tubes with rigid 
walls. The elasticity, moreover, assists in the equalization of the blood 
stream, converting the intermittent and remittent flow characteristic of the 
large arteries into the continuous equable stream characteristic of the cap- 
illaries. It also permits of wide variations in the amount of blood the 
arteries can contain between their minimum and maximum distention. 

Variations in the Arterial Pressure. — From the preceding statements 
it is apparent that the existing arterial pressure may be increased by: 
i. An increase in the rate or force of the heart's contraction. 

2. An increase in the peripheral resistance. 

3. An increase in both the force of the heart and the peripheral resist- 
ance; and it is further apparent that if the pressure is higher than the nor- 
mal it may be lowered to the normal by a decrease in either one or both of 
these factors. 

It is also apparent that the arterial blood-pressure as a whole may be 
decreased below the normal by: 

1. A decrease in the rate and force of the heart's contraction. 

2. A decrease in the peripheral resistance. 

3. A decrease in both the force of the heart and the peripheral resistance; 
and it is again further apparent, that if the pressure is lower than the nor- 
mal it may be raised to the normal by an increase in either one or both of 
these factors. 

The Capillary Pressure. — The capillary pressure, though possessing an 
average value, may be increased by a relaxation of the arteriole muscle and 
decreased by their contraction. It may also be increased by any inter- 
ference with the outflow from any given area, or decreased by factors which 
favor a larger outflow. Independently of any change in arteriole resist- 
ance, a rise of arterial pressure alone will increase the capillary pressure. 

The Venous Pressure. — The venous pressure as a whole will be in- 
creased by a fall in arterial pressure as when the arterioles relax and the 
heart diminishes in force; it will be decreased if the opposite factors 
prevail. 

The Pulse. — The pulse may be defined as a periodic expansion and recoil 
of the arterial system. The expansion is caused by the ejection into the 
arteries of a volume of blood during the systole; the recoil is due to the 
reaction of the arterial walls on the blood driving it forward into and 
through the capillaries, during the diastole. 



CIRCULATION OF THE BLOOD II9 

At the close of the cardiac diastole the arteries are full of blood and con- 
siderably distended. During the occurrence of the succeeding systole, the 
incoming volume of blood is accommodated by a movement forward of a 
portion of the general blood volume into the capillaries and a further dis- 
tention of the arteries. The distention naturally begins at the beginning 
of the aorta. As the blood continues to be discharged from the heart, ad- 
joining segments of the aorta expand in cjuick succession and by the end of 
the systole the expansion has travelled over the arterial system as far as the 
capillaries. This expansion movement which passes over the arterial 
system in the form of a wave is known as the pulse wave, or the pulse. It 
is this alternate expansion and recoil which is perceived by the finger when 
placed over the course of an artery. The artery best adapted for this 
purpose is the radial as it passes across the wrist-joint. 

The Radial Pulse. — If the ends of the fingers are firmly placed over the 
radial artery, not only the increase and decrease of pressure, but also many 
of the peculiarities of the pulse-wave, may be perceived. Without much 
difficulty it may be perceived that the expansion takes place quickly, the 
recoil relatively slowly; that the waves succeed one another with a certain 
frequency, corresponding to the heart-beat; that the pulsations are rhyth- 
mic in character, etc. 

Inasmuch as the individuality of the pulse-wave varies at different 
periods of life and under different physiologic and pathologic conditions, 
various terms more or less expressive, have been suggested for its varying 
qualities. Thus the pulse is said to be frequent or infrequent according as 
it exceeds or falls short of a certain average number — 72 per minute; 
strong or weak according to the energy with which the vessel expands; 
quick or slow, according to the suddenness with which the expansion takes 
place or strikes the fingers; hard or soft, tense or easily compressible, accord- 
ing to the resistance which the vessel offers to its compression by the fingers; 
large, full or small, according to the volume of blood ejected into the aorta, 
or, in other words, the degree of fullness of the arterial system. 

The three qualities which are of most value to the clinician are rate, 
ngth or force, and volume. 

The Velocity of the Blood. — The velocity with which the blood flows in 
the arteries dimi:.' m the heart to the capillaries, owing to an en- 

largement in the united sectional area of the vessels; the velocity increases 
from the capillaries toward the heart for the opposite reason. The blood 
moves most rapidly in the large vessels, and especially under the influence 
of the ventricular systole. From experiments on animals, it has been es- 



120 HUMAN PHYSIOLOGY 

timated to move in the aorta of man at the rate of from 300 to 500 mm. 
a second, and in the large veins at the rate of from 150 to 250 mm. a second, 
and in the capillaries from 0.5 to 1 mm. per second. 

The Pulmonic Vascular Apparatus. — The pulmonic vascular apparatus 
consists of a closed system of vessels extending from the right ventricle 
to the left auricle, and includes the pulmonic artery, capillaries, and pul- 
monic veins. In its anatomic structure and physiologic properties it 
closely resembles the systemic apparatus. 

The flow of the blood through the arteries, capillaries and veins is 
characterized by velocity and pressure and in the pulmonic artery 
alone by the presence of the pulse. The causes of the'se phenomena in the 
pulmonic vascular apparatus are the same as in the systemic apparatus. 
The pressure in the pulmonic artery of the dog has been shown by Beutner 
to be about one-third that in the aorta; by Bradford and Dean to be one- 
fifth. Wiggers has recently shown that in normally breathing dogs with 
arterial pressures ranging from no to 112 mm. of mercury, the maximal 
or systolic pressure in the pulmonic artery averaged 36 mm., and the 
minimal or diastolic averaged 5 mm. The reason for the low pressure 
may be found in the large size and rich development of the pulmonic 
capillaries and the imperfect development of an arteriole muscle at the 
periphery of the pulmonic artery, the result of which is a diminution 
in the friction. Inasmuch as the friction is relatively low, the work of 
the right heart is less than that of the left heart and hence its walls are 
not so well developed. The pulmonic pressure being low the intra- 
ventricular pressure of the right heart is relatively low as compared with 
that of the left heart. The velocity of the blood-stream in each of the 
three divisions of the system cannot well be determined. The time oc- 
cupied by a particle of blood in passing from the right to the left ventricle 
has been estimated at one-fourth the time required to pass from the left to 
the right ventricle. Assuming the latter to be thirty seconds, the former 
would be seven and one-half seconds. 

The capillary vessels are spread out in a very elaborate manner just 
beneath the inner surface of the pulmonic air cells, and form, by their 
close relation to it, a mechanisms for the excretion of carbon dioxid and the 
absorption of oxygen. The extent of the capillary surface is very great. 
It has been estimated at 200 square meters. The amount of blood flowing 
through this system hourly and exposed to the respiratory surface is about 
430 liters. The reason for the existence of the pulmonary circulation is the 
renewal of the oxygen in the blood and the elimination of the carbon dioxid; 
for the accomplishment of both objects ample provision is here made. The 



CIRCULATION OF THE BLOOD 12 1 

flow of blood through the cardio-pulmonic vessels is subject to variation 
during both inspiration and expiration in consequence of their relation to 
the respiratory apparatus. 

Forces Concerned in the Circulation of the Blood : 

i. The Contraction of the Heart. — The primary forces which keep the 
blood flowing from the beginning of the aorta to the right side of the heart 
and from the beginning of the pulmonary artery to the left side are the 
contractions of the left and right ventricles respectively. Though the 
heart's energy is probably sufficient to drive the blood into the opposite 
side of the heart, it is supplemented by other forces — e.g. : 

2. Muscle Contraction. # 

3. Thoracic Aspiration. 

4. The Action of the Valves in the veins. 

The Vaso-motor Nerves. — These are nerves that impart motor activity 
to the muscle-fibers of the arteriole walls, resulting either in an increase or 
decrease in the degree of their contraction and thus diminishing or in- 
creasing the outflow of blood. For this reason they are termed vaso- 
augmentor or constrictor nerves and vaso-inhibitor or dilatator nerves. 

- the muscle-fibers belong to the autonomic tissues, the nerve supply to 
them consists of two consecutively arranged neurons, a pre-ganglionic and 
a post-ganglionic. 

The pre-ganglionic vaso-constrictor neurons take their origin from nerve- 
cells located in the anterior horns and lateral gray matter of the spinal 
cord. They emerge from the cord in company with the fibers that com- 
the ventral roots of the spinal nerves from the second thoracic to the 
second or third lumbar nerves inclusive. A short distance from the cord 
they leave the ventral roots as the white rami communicantes and enter 
1 for the most part the vertebral or lateral sympathetic ganglia. From the 
Its of many observations and experiments it is probable that the great 
majority of the vaso-constrictor nerves terminate in these ganglia; that is 
• is here that the pre-ganglionic fibers arborize around the con- 
tainer! n< From the nerve-cells the post-ganglionic fibers arise, 
which pass to the blood-vessels of (1) the body walls; (2) the fore-limbs; 
(3) the h< - and face; (4) the hind limbs; and (5) the abdominal 
ra. 
The fibers for the blood-vessels of the abdominal viscera and which are 
1 contained in the trunk of the splanchnic nerve pass across the sympathetic 
1 chain and arb indthene milunar ganglion. The 
post-ganglionic arise from the cellfl of thi^ ganglion and then pass to the 
blood-vessels of the stomach, intestines, liver, etc. 

) 



122 HUMAN PHYSIOLOGY 

The Vaso-motor Centers. — The vaso-motor centers for the spinal cord 
are dominated and controlled in their action by a group of nerve-cells in 
the floor of the fourth ventricle which is known as the general vaso-con- 
strictor center. This center is supposed to consist of two groups of cen- 
ters, viz., a vaso- tonic and a vaso-reflex center; the former maintains the 
vascular tonus while the latter permits of various vaso-motor reflexes. 

The vaso-reflex center may be increased or decreased in its activity by 
nerve impulses transmitted to it from different regions of the body, in 
consequence of which the blood distributed to larger or smaller areas of 
the body is decreased or increased in accordance with their physologic 
needs. 

Special vaso-dilatator centers are found in the medulla, for the blood- 
vessels of the glands of the mouth, nasal chambers, etc. ; also in the lower 
lumbar region of the cord there are centers for the blood-vessels of the 
sexual organs. Stimulation of these centers, either reflexly, or directly 
from the cerebrum, causes dilatation of the vessels and a large inflow of 
blood. 

RESPIRATION 

Respiration is a process by which oxygen is introduced into, and carbon 
dioxid removed from the body. The assimilation of the former and the 
evolution of the latter take place in the tissues as a part of the general 
process of nutrition. Without a constant supply of oxygen and an equally 
constant removal of the carbon dioxid, those chemic changes which under- 
lie and condition of life phenomena could not be maintained. 

The general process of respiration may be considered under the following 
headings, viz.: 

i. The anatomy and general arrangement of the respiratory apparatus. 

2. The mechanic movements of the thorax by which an interchange of 
atmospheric and intra-pulmonary air is accomplished. 

3. The chemistry of respiration; the changes in composition undergone 
by the air, blood, and tissues. 

4. The nerve mechanism by which the respiratory movements are 
maintained and coordinated. 

The Respiratory Apparatus. — The respiratory apparatus consists essen- 
tially of: 

1. The lungs and the air-passages leading into them: viz., the nasal 
chambers, mouth, pharynx, larynx, and trachea. 

2. The thorax and its associated structures. * 



RESPIRATION 1 23 

The Larynx. — The larynx is composed of firm cartilages, united by liga- 
ments and muscles. Running anteroposteriorly across the upper opening 
are four ligamentous bands — the two superior or false vocal bands, and the 
two inferior or true vocal bands — formed by folds of the mucous mem- 
brane. They are attached anteriorly to the thyroid cartilages and pos- 
teriorly to the arytenoid cartilages, and are capable of being separated by 
the contraction of the posterior crico-arytenoid muscles, so as to admit 
the passage of air into and from the lungs. 

The Trachea. — The trachea is a tube from 10 to 12 centimeters in length, 
2 centimeters in diameter, extending from the cricoid cartilage of the 
larynx to the fifth thoracic vertebra, where it divides into the right and 
left bronchi. It is composed of a series of cartilaginous rings, which extend 
about two-thirds around its circumference, the posterior third being cc- 
cupied by transversely arranged non-striated muscle-fibers known as the 
tracheal muscle. Being attached to the ends of the cartilages it is capable, 
by alternately contracting and relaxing, of diminishing or increasing the 
lumen of the trachea. Opposite the fifth thoracic vertebra the trachea 
divides into a right and left bronchus. Each bronchus then subdivides 
into two other branches which penetrate the corresponding lung about the 
middle of the inner surface. 

The Lungs. — The lungs, in the physiologic condition, occupy the greater 
part of the cavity of the thorax. They are separated from each other by 
the contents of the mediastinal space: viz., the heart, the large blood- 
vessels, the esophagus, etc. 

A histologic analysis of the lung shows it to consist of the branches of the 
bronchi, their subdivisions and ultimate terminations, blood-vessels, 
lymphatics and nerves, imbedded in a stroma of fibrous and elastic tissue. 
The anatomic relations which these structures bear one to another is as 
follows: 

Within the substance of the lung the bronchi divide and subdivide, giving 
origin to a large number of smaller branches, the bronchial tubes, which pen- 
etrate the lung in all directions (Fig. 12). With this repeated subdivision 
the tubes become narrower, their walls thinner, their structure simpler. 
In passing from the larger to the smaller tubes the cartilaginous arches 
become shorter and thinner, and finally are represented by small angular 
and irregularly disposed plates. In the smallest tubes the cartilage en- 
tirely disappears. With the diminution of the caliber of the tube and a 
decrease in the thi< kncss of its walls, there appears a layer of non-striated 
muscle-fibers, the so-called bronchial muscle, between the mucous and sub- 
mucous tissues, which completely surrounds the tube and becomes 



124 



HUMAN PHYSIOLOGY 



especialUy well developed in those tubes devoid of cartilage. The fibrous 
and mucous coats at the same time diminish in thickness. 

When the bronchial tube has been reduced to the diameter of about one 
millimeter, it is known as a bronchiole or a terminal bronchus. From the 
sides of the terminal bronchus and from its final termination there is given 
off a series of short branches which sooxpan end to form lobules or alveoli. 
The cavity of the alveolus is termed the infundibulum. From the inner 
surface of the alveolus and of the passageway leading into it, there project 

thin partitions which subdivide the 
outer portion of the general cavity or 
infundibulum into small spaces, the 
so-called air-sacs or air-cells. The wall 
of the alveolus is extremely thin and 
consists of fibro-elastic tissue, support- 
ing a very elaborate capillary net- 
work of blood-vessels. The bronchial 
system as far as the alveolar passages is 
lined by ciliated epithelum. The air- 
sacs are lined by flat epithelial plates 
of irregular shape, termed the respira- 
tory epithelium. The alveoli are united 
one to another by fibro-elastic tissue. 

Bronchial Innervation. — The bron- 
chial muscles are presumably in a state 
of tonic contraction and impart to the 
bronchial tubes a certain average caliber 
best adapted for respiratory purposes. 
Experimental investigations indicate 
that they are innervated by efferent 
fibers of the vagus nerve (broncho- 
constrictors and possibly broncho-dila- 
tators) inasmuch as stimulation of this nerve is usually followed by a con- 
traction of the muscles and a narrowing of the lumen of the bronchial 
system. These muscles may also be thrown into increased activity by 
the inhalation of irritating gases and into a tetanus by pathologic causes 
as seen in the various forms of asthma. 

The Pulmonic Blood-vessels. — The two main divisions of the pulmonic 
artery distribute the venous blood to the pulmonic lobules. As the lobules 
are approached a small arterial branch plunges into the wall of the lobule, 
in which it branches form a rich capillary network in which surrounds and 




Fig. 12. — Diagram of the Re- 
spiratory Organs. 
The windpipe, leading down 
from the larynx, is seen to branch 
into m two large bronchi, which 
subdivide after they enter their 
respective lungs. 



RESPIRATION 1 25 

embraces the air sacs on all sides. The blood -emerging from the capil- 
laries is conducted by the converging system of veins — the pulmonic 
veins — into the left auricle of the heart. The main function of the pul- 
monic appparatus and the pulmonic division of the circulatory apparatus 
is to afford a read}* means for the exhalation of the carbon dioxid and the 
absorption of oxygen. In consequence of this exchange of gases the 
blood changes in color from dark bluish-red to scarlet red. 

The Thorax. — The thorax in which the respiratory organs are lodged, is 
of a conic shape, having its apex directed upward, its base downward. Its 
framework is formed posteriorly by the spinal column, anteriorly by the 
sternum, and laterally by the ribs and costal cartilages. Between and 
over the ribs lie muscles, fascia, and skin; above, the thorax is completely 
closed by the structures passing into it and by the cervical fascia and skin; 
below, it is closed by the diaphragm. It is, therefore, an air-tight cavity. 

The Pleura. — Each lung is surrounded by a closed serous membrane 
(the pleural one layer of which (the visceral) is reflected over the lung; 
the other (the parietal), reflected over the wall of the thorax; between the 
two layers is a small amount of fluid, which prevents friction during the 
play of the lungs in respiration. 

The Relation of the Respiratory Organs. — Intra- pulmonic pressure. — 
When the thorax is in a condition of rest, as at the end of an expiration the 
lungs are full of air and by reason of their distensibility completely fill all 
portions of the thorax not occupied by the heart, great vessels, and esopha- 
gus. This condition is maintained by the pressure of the air in the lungs, 
the intra- pulmonic pressure, which is that of the atmosphere 760 mm. Hg. 
This relation persists so long as the thorax remains air tight. If however 
an opening be made in the thoracic wall, the lung immediately collapses 
and a pleural cavity is established. The pressure of air within and without 
the lung counterbalancing, at the moment the air is admitted, the elastic 
tissue at once recoils and forces a large part of the air out of the lung. 
This is a proof that in the normal condition, the lungs, distended by atmos- 
pheric pressure from within, are in a state of elastic tension and eser en- 
deavoring to pull the pulmonic layer of the pleura away from the parietal 
layer. That they do not succeed in doing so is due to the fact that the 
atmospheric pressure from without is prevented from acting on the lung 
by the firm unyielding walls of the thorax. 

Intra-!'. >>:ssurc. — As a result of the elastic tension of the Iuiil: 

fractional part of the intra-pulmonic pressure, 760 mm. Hg., is counter- 
balanced or opposed, so that the heart and great vessels and other intra- 
thoracic viscera are Subjected to a pressure somewhat less than that of the 



126 HUMAN PHYSIOLOGY 

atmosphere; the amount of this pressure will be that of the atmosphere 
less that exerted by the elastic tissue of the lung in the opposite direction, 
expressed in terms of millimeters of mercury. In the thorax but outside 
the lungs, there then prevails a pressure, negative to the pressure inside the 
lungs and which is known as the intra-thoracic pressure. 

The elastic tension of the lung has been determined for the human lung 
and amounts to about 6 mm. Hg. The intra-thoracic pressure is nega- 
tive to the intra-pulmonic pressure by 6 mm. Hg. 

The Respiratory Movements. — As the blood flows through the pul- 
monic capillaries it yields carbon dioxid to, and receives oxygen from, the 
air in the pulmonic alveoli. As a result, the intra-pulmonic air changes in 
composition, which interferes to a greater or less extent with the further ex- 
change of gases. That this exchange may continue, it is of primary impor- 
tance that the air within the alveoli be renewed as rapidly as it is vitiated. 
This is accomplished by an alternate increase and decrease in the capacity of 
the thorax, accompanied by corresponding changes in the capacity of the 
lungs. During the former there is an inflow of atmospheric air (inspira- 
tion), during the latter an outflow of intra-pulmonic air (expiration). 
The continuous recurrence of these two movements brings about that de- 
gree of pulmonic ventilation necessary to the normal exchange of gases 
between the blood and the air. The two movements together constitute 
a respiratory act or cycle. 

i. Inspiration is an active process, the result of the expansion of the 
thorax, whereby the atmospheric air is introduced into the lungs. 

2. Expiration is a partially passive process, the result of the recoil of the 
elastic walls of the thorax, and the recoil of the elastic tissue of the lungs 
whereby the intrapulmonary air is expelled. 

In inspiration the chest is enlarged by an increase in all its diameters — 
viz.: 

i. The vertical is increased by the contraction and descent of the 
diaphragm. 

2. The anteroposterior and transverse diameters are increased by the ele- 
vation and rotation of the ribs upon their axes. 

In ordinary tranquil inspiration the muscles which elevate the ribs and 
thrust the sternum forward, and so increase the diameters of the chest, are 
the external inter costals, running from above downward and forward; the 
sternal portion of the internal intercostals, and the levatores costarum. 

In the extraordinary efforts of inspiration certain auxiliary muscles are 
brought into play — viz., the sternotnastoid, pectorales, serratus magnus — 
which increase the capacity of the thorax to its utmost limit. 



RESPIRATION 1 27 

In expiration the diameters of the chest are all diminished — viz.: 

1. The vertical, by the ascent of the diaphragm. 

2. The anteroposterior, by a depression of the ribs and sternum. 

In ordinary tranquil expiration the diameters of the thorax are diminished 
by the recoil of the elastic tissue of the lungs and the ribs; but in forcible 
expiration the muscles which depress the ribs and sternum, and thus further 
diminish the diameter of the chest, are the internal intercostal s, the infra- 
costal s, and the triangularis stcrni. 

In the extraordinary efforts of expiration certain auxiliary muscles are 
brought into play — viz., the abdominal and sacrolumbalis muscles — which 
diminish the capacity of the thorax to its utmost limit. 

The Movements of the Lungs. — By reason of the distensibility and the 
elastic recoil of the lungs, they follow all variations in the size of the thorax 
enlarging during inspiration to accommodate the incoming volume of air 
and diminishing during expiration to assist in the removal of a correspond- 
ing amount of air. 

During the enlargement of the thorax, the intra-pulmonic air expands 
and its pressure falls in consequence of which the atmospheric air rushes in 
to restore atmospheric pressure. Coincidently the lungs are expanded and 
kept in close contact with the thoracic walls and the diaphragm. During 
the diminution in the size of the thorax, the intra-pulmonic air is com- 
pressed and its pressure rises, in consequence of which the intra-pulmonic 
air rushes out through the air passages until the atmospheric pressure is 
reached. Coincidently the elastic recoil of the lungs restores them to their 
former size and volume. 

The intra-thoracic pressure falls during inspiration and rises during ex- 
piration. The expansion of the lungs is attended by an increase in the 
elastic recoil and hence a neutralization of a larger percentage of the intra- 
pulmonic pressure. The recoil of the lungs during expiration has the oppo- 
site result. 

The fall of intra-thoracic pressure has a favorable influence on the flow of 
blood from the extra-thoracic veins into the intra-thoracic veins, the right 
side of the heart and the cardio-pulmonic vessels. The flow of lymph from 
the lower portion of the thoracic duct into the upper portion is also in- 
creased. During expiration the reverse movement is prevented by the 
action of the valves. 

Types of Respiration. — Observations of the respiratory movements in 
the two sexes shows that while the enlargement of the thoracic cavity is 
accomplished both by the descent of the diaphragm (as shown by the pro- 
trusion of the abdomen) and the elevation of the thoracic walls, the former 



128 HUMAN PHYSIOLOGY 






movement preponderates in the male, the latter in the female, giving rise 
to what has been termed in the one case the diaphragmatic or abdominal 
type and in the other the thoracic or costal type of respiration. Modern 
methods of investigations have established the view that the preponder- 
ance of thoracic movement is due to the influences of dress restrictions, 
for with their removal the so-called costal type of breathing entirely 
disappears. While gestation may lead to a greater activity of the thorax, 
this is but temporary, for with its termination there is a return to the dia- 
phragmatic type of breathing. 

Number of Respirations per Minute. — The number of respirations which 
occur in a unit of time varies with a variety of conditions, the most impor- 
tant of which is age. The* results of the observations of Quetelet on this 
point, which are generally accepted, are as follows: 

Respirations Respirations 

Age. per minute Age per minute 

o- i year 44 20-25 years 18.7 

5 years 26 25-30 years 15. o 

15-20 years 20 30-50 years 17.0 

From these observations it may be assumed that the average number of 
respirations in the adult is eighteen per minute, though varying from 
moment to moment from sixteen to twenty. During sleep, however, the 
respiratory movements often diminish in number as much as 30 per cent., 
at the same time diminishing in depth. 

Volumes of Air Breathed. — The volumes of air which enter and leave 
the lungs with each inspiration and expiration naturally vary with extent 
of the movement, though four volumes at least, may be determined: 
(1) that of an ordinary inspiration; (2) that of an ordinary expiration; 
(3) that of a forced inspiration; (4) that of a forced expiration. 

By means of the spirometer the amount of the foregoing four volumes 
have been determined and named as follows : 

1. The tidal volume, that which flows into and out of the lungs with each 
inspiration and expiration, which varies from 20 to 30 cubic inches (330 to 
500 c.c). 

2. The complemental volume, that which flows into the lungs, in addition 
to the tidal volume, as a result of & forcible inspiration, and which amounts 
to about no cubic inches (1,800 c.c). 

3. The reserve volume, that which flows out of the lungs, in addition to 
the tidal volume, as a result of a forcible expiration, and which amounts to 
about 100 cubic inches (1,650 c.c). 

After the expulsion of the reserve volume there yet remains in the lungs 



RESPIRATION 1 29 

an unknown volume of air which serves the mechanic function of distend- 
ing the air-cells and alveolar passages, thus maintaining the conditions 
essential to the free movement of blood through the capillaries and to the 
exchanges of gases between the blood and alveolar air. As this volume of 
air cannot be displaced by volitional effort, but resides permanently in the 
alveoli and bronchial tubes though constantly undergoing renewal, it was 
termed — 

4. The residual volume, the amount of which is difficult of determina- 
tion, but has been estimated by different observers at 914 c.c. 1,562 c.c, 
1,980 c.c. 

The Vital Capacity of the Lungs. — The total volume of the air in the 
lungs at the time of their maximum distention represents the vital capacity 
in the physiological condition and includes the tidal, the complemental, 
the reserve and the residual air. The vital capacity, however, has been 
defined as the amount of air which can be expelled by the most forcible 
expiration after the most forcible inspiration, this therefore excludes the 
residual volume. The vital capacity was supposed to be an indication of 
an individual's respiratory power, not only in physiologic but also in patho- 
logic conditions. Though averaging about 230 cubic inches (3,770 c.c.) 
for an individual 5 feet 7 inches in height, the vital capacity varies with a 
number of conditions, the most important of which is stature. It is 
found that between 5 and 6 feet the capacity increases 8 inches (130 c.c.) 
for each inch increase in height. 

The total volume of air breathed daily can be approximately determined 
by multiplying the average volume of air taken in at one inspiration and 
multiplying by the number of respirations per minute. Assuming that 
an individual takes into the lungs at each inspiration 330 to 500 c.c. (20 
to 30 cubic inches) and at the same time breathes 18 times per minute 
there would pass into the lungs during the twenty-fours, 8,500 to 12,752 
liters. 

The Chemistry of Respiration. Changes in the Composition of the 
Air Breathed. — Experience teaches that the air during its sojourn in the 
lungs undergoes such a change in composition that it is rendered unfit for 
further breathing. Chemic analysis has shown that this change involves 
a loss of oxygen, a gain in carbon dioxid, watery vapor, and organic matter. 
For the correct understanding of the phenomena of respiration it is essen- 
tial that not only the character but the extent of these changes be known. 
This necessitates an analysis of both the inspired and expired airs, from a 
comparison of which certain deductions can be made. 
I 9 



130 HUMAN PHYSIOLOGY 

The results which have been obtained are represented in the following 
table: 

Inspired air Expired air 

Oxygen 16.02. 

Carbon dioxid 4.38. 

Nitrogen 79 .60. 

Watery vapor saturated. 

Organic matter a trace. 



100 
vols. 



Oxygen 20.80. 

Carbon dioxid traces. 

Nitrogen. 79.20. 

Watery vapor variable. 

\ 



100 

vols. 



These analyses indicate that under ordinary conditions the air loses 
oxygen to the extent of 4.78 per cent, and gains carbon dioxid to the extent 
of 4.38 per cent.; that it gains in nitrogen to the extent of 0.4 per cent, and 
in watery vapor from its initial amount to the point of saturation, as welt 
as in organic matter. It is to these changes in their totality that those 
disturbances of physiologic activity are to be attributed which arise when 
expired air is re-breathed for any length of time without having undergone 
renovation. From the percentage loss of oxygen and gain in carbon dioxid 
the total oxygen absorbed and carbon dioxid exhaled may be approximately 
calculated. Thus, if the volume of air breathed daily be accepted at either 
8,500 or 12,752 liters, and the percentage loss of oxygen be 4.78, the total 
oxygen absorbed may be obtained by the rule of simple proportion, e.g.: 

100 : 4.78 : : 8,500 : * = 406 liters or 580 grams 1 
Or 

100 : 4.78 : : 12,752:* = 609 liters or 870 grams. 

By the same method the total carbon dioxid exhaled is found to be either 
372 liters or 735 grams, or 558 liters or 1,103 grams, volumes in both in- 
stances which agree very well with volumes obtained by other methods. 

As there is always more oxygen consumed than carbonic acid exhaled, 
and as oxygen unites with carbon to form an equal volume of carbonic acid, 
it is evident that a certain quantity of oxygen disappears within the body. 
In all probability it unites with the surplus hydrogen of the food to form 
water. 

The quantities both of oxygen absorbed and carbon-dioxid exhaled 
daily is subject to considerable variation. They are increased by exercise, 
digestion and a lowered temperature, and decreased by the opposite 
conditions. 

The gain in watery vapor will depend on the amount previously present 
in the air. This is conditioned by the temperature. With a rise in tem- 
perature the percentage of water increases; with a fall, it decreases. 

1 1 liter of oxygen weighs 1.4298 grams; 1 liter of carbon dioxid weighs 1.977 grams. 



RESPIRATION 131 

The gain in organic matter is also variable. The amount present is not 
sufficient to permit of a thorough chemic analysis, but there are reasons for 
believing that it belongs to the protein group of bodies. If it accumulates 
in the air, especially at high temperatures, it readily undergoes decomposi- 
tion, with the production of offensive odors. Traces of free ammonia have 
also been found in the expired air. In addition to these chemic changes, 
the air experiences physical changes; e.g., a rise in temperature and an in- 
crease in volume. The rise in temperature can be shown by breathing 
through a suitable mouthpiece into a glass tube containing a thermometer. 
By this means it has been shown that inspired air at 2o°C. rises in tempera- 
ture to 37°C; at 6.3 to 2o.8°C. The increase in the temperature will 
depend upon that of the air inspired and the time it remains in the lungs. 
If retained a sufficient length of time it will always become that of the 
body. 

Changes in the Composition of the Blood. — As the blood of the pulmonic 
artery passes through the pulmonic capillaries, it loses carbon dioxid and 
gains oxygen, in consequence of which, it changes in color from a bluish red 
to a scarlet red. As the blood of the systemic arteries flows into and 
through the systemic capillaries it loses oxygen and gains carbon dioxid 
in consequence of which it changes in color from a scarlet red to a bluish 
red. 

The Gases of the Blood. — The presence of gases in the blood is demon- 
strated by subjecting it to the vacuum of the air pump into which they at 
once escape. 

An analysis of the gases so obtained gives the following results. 

- . .... j f Oxygen 20 vols. , r ,, , f Oxygen 12 vols. 

Arterial blood J t a- a 1 Venous blood J 7 * 

\ Carbon dioxid. . 40 vols. . < Carbon dioxid. 45 vols. 

100 vols. *... , 100 vols. HO 

[Nitrogen 1-2 vols. [Nitrogen 1-2 vols. 

The changes produced in the blood by respiration, both external and inter- 
nal, become apparent from a comparison of these analyses. The arterial 
blood while passing through the capillaries of the tissues loses eight vol 
umes per cent, of oxygen and gains five per cent, of carbon dioxid. The 
venous blood while passing through the capillaries of the lungs gains 
oxygen and loses carbon dioxid in corresponding amounts. These amounts 
will vary somewhat in the analyses of the blood of different animals and 
under different physiologic conditions. The volume of nitrogen is not 
appreciably changed. 

The Condition of the Gases in the Blood. — After the oxygen of the alve- 
oli passes across the thin alveolo-capillary wall into the blood it combines 
with hemoglobin to form oxyhemoglobin, the compound that gives the 



132 HUMAN PHYSIOLOGY 

scarlet-red color to the arterial blood. As the arterial blood flows into 
the capillary vessels the oxygen is in part dissociated from the hemoglobin 
and passes across the capillary wall into the tissues. 

The carbon dioxid arising in the tissues passes across the capillary wall 
into the blood where a portion of it is physically absorbed, while another 
portion combines with sodium carbonate to form a bicarbonate. As the 
blood passes through the pulmonic capillaries the carbon dioxid is in part 
dissociated and then passes across the alveolo-capillary wall into the 
interior of the alveoli. 

Changes in the Composition of the Tissue Fluids. — An analysis of the 
tissue fluids shows the absence of oxygen and the presence of carbon dioxid. 
Notwithstanding the continuous passage of oxygen across the capillary 
walls into the tissue fluids free oxygen cannot be determined in them. 
The absence of oxygen would indicate that it is immediately ultilized by 
the tissue with the production of carbon dioxid; or that it is stored in the 
tissues in some form or other by which it can be retained until required for 
oxidation purposes — the latter is the more likely view. 

The carbon dioxid is present in variable quantities in the tissues and 
fluids and though passing into the blood at varying rates it is as constantly 
being evolved. 

The Mechanism of the Gaseous Exchange. — The passage of the oxygen 
from the alveoli into the blood and into the tissues, and the passage of the 
carbon dioxid from the tissues into the blood and into the alveoli is believed 
to be due to differences of pressure. In the alveoli the oxygen pressure is 
approximately equal to 130 mm. of Hg.; in the arteries 106 mm. Hg. and 
in the tissues zero. In the tissues the carbon-dioxid pressure varies from 
45 to 68 mm. of Hg.; in the veins 42 mm. Hg. and in the alveoli about 38 
mm. Hg. In these differences of pressure is to be found an explanation for 
the exchange of these gases. 

The Total Respiratory Exchange. — The total quantities of oxygen ab- 
sorbed and carbon dioxid discharged by a human being in twenty-four 
hours are measures of the intensity of the respiratory process, and an indi- 
cation of the extent and character of the chemic changes attending all life 
phenomena. 

Approximate amounts of oxygen absorbed and carbon dioxid exhaled 
as determined by different investigators are as follows: 

Oxygen absorbed Observer Carbon dioxid discharged 

746 grams. Vierordt. 876 grams. 

700 grams. Pettenkofer and Voit. 800 grams. 

663 grams. Speck. 770 grams. 



RESPIRATION 1 33 

The amounts of oxygen absorbed in Pettenkofer and Voit's experiments 
varied from 594 to 1,072 grams; of carbon dioxid exhaled, from 686 to 
1,285 grams. 

The Nerve Mechanism of Respiration. — The simultaneous and coordi- 
nated activity of the inspiratory muscles implies the simultaneous and 
coordinated activity of nerve centers and their related motor nerves. 
Thus the action of the nasal and laryngeal muscles (the dilatator naris and 
the posterior crico-arytenoid) involves the activity of the facial and infe- 
rior laryngeal nerves respectively, the centers of origin of which lie in the 
gray matter beneath the floor of the fourth ventricle; the diaphragm and 
intercostal muscles involve respectively the activity of the phrenic and 
intercostal nerves, the centers of origin of which lie in the anterior horn of 
the gray matter of the spinal cord at a level, for the phrenic, of the fourth, 
fifth, and sixth cervical nerves, and for the intercostals at the level of the 
thoracic nerves. Division of any one of these nerves is followed by 
paralysis of its related muscle. 

Inspiratory Center. — The coordinate contraction of the inspiratory 
muscles implies a practically simultaneous discharge of nerve impulses 
from each of the foregoing nerve-centers, accurately graduated in intensity 
in accordance with inspiratory needs. This has been supposed to necessi- 
tate the existence in the central nerve system of a single group of nerve- 
cells from which nerve impulses are rhythmically discharged and conducted 
to the previously mentioned nerve-centers in the medulla oblongata and 
spinal cord, by which they are in turn excited to activity. To this group 
of cells the term "inspiratory center" has been given. 

The rhythmic activity of the inspiratory center is in part the result of the 
stimulating action of carbon dioxid and partly the result of the trans- 
mission to it of nerve impulses from various regions of the body. The 
irritability of the center is markedly increased by the percentage of carbon 
dioxid in the blood and decreased by the opposite condition. The vagus 
nerves of all afferent nerves are the most influential in maintaining the 
normal rhythmic discharge of nerve impulses from the inspiratory center 
1 as shown by the effects that follow their separation from the center. 
Thus, if while the animal is breathing regularly and quietly both vagi are 
cut, the respiratory movements become much slower, falling perhaps to one- 
third their original number per minute. At the same time the inspirations 
i become deeper and somewhat spasmodic in character. The duration of 
1 the inspiratory movement is also increased beyond that of the expiratory 
movement. If now the central end of one of the divided vagi be stimu- 
lated with weak induced electric currents, the respiratory movements are 



134 HUMAN PHYSIOLOGY , 

again increased in frequency, and their depth diminished until the normal 
rate is restored. With the cessation of the stimulation the former"condi- 
tion at once returns. This would seem to indicate that the vagus nerve 
contains nerve-fibers which, under physiologic conditions, transmit 
nerve impulses which inhibit the inspiratory discharge and lead to an ex- 
piratory movement sooner than would otherwise be the case, and thus 
maintain the normal rate and extent of the inspiratory discharge. 

Stimulation of the central end of the divided vagus with strong electric 
currents excites the activity of the inspiratory center to such an extent, 
that the muscles pass into the tetanic state and the thorax comes to rest in 
the condition of a forced inspiration. 

These results indicate that the vagus nerve contains two classes of 
fibers which influence the activity of the inspiratory center, viz. : an exci- 
tator and an inhibitor. The stimulus to their excitation is to be found in 
the alternate recoil and expansion of the alveoli, in the walls of which, they 
terminate. With the recoil of the alveolar walls nerve impulses are de- 
veloped which ascend the vagi to the inspiratory center and excite it to 
activity and thus call forth a new inspiratory movement sooner than it 
would otherwise take place. With the expansion of the alveoli, nerve 
impulses are developed which ascend the vagi to the inspiratory center and 
inhibit its activity and thus lead to an expiratory movement sooner than it 
would otherwise take place. The respiratory mechanism is apparently 
self-regulative and maintained by the alternate recoil and expansion of the 
lungs. 

The Establishment of Respiration after Birth. — During intra-uterine 
life the exchange of gases is accomplished by the placenta. Immediately 
after birth, this method is abolished. The cause of the first inspiration 
therefore must be associated with an increase in the percentage of carbon 
dioxid or a decrease in the percentage of oxygen in the blood. The former 
condition is more likely to be the efficient cause. The rapid accumulation 
of carbon dioxid with its increasing pressure in the inspiratory center so 
raises its irritability, as to lead to a discharge of nerve impulses which are 
conducted to the inspiratory muscles and cause their contraction. With 
the first inspiration thus established the nerve mechanism comes into 
play. 

Inasmuch as cold water applied to the skin of the adult profoundly ex- 
cites at times the inspiratory center it has been assumed that an additional 
factor leading to an excitation of the inspiratory center is the rapid cooling 
of the surface of the child by the evaporation of the amniotic fluid from 
the surface of the skin. The nerve impulses thus developed are trans- 
mitted through cutaneous nerves to the inspiratory center. This assump- 



ANIMAL HEAT 135 

tion is somewhat strengthened by the fact that in delayed inspiration the 
stimulation of the skin by the application of cold water frequently leads to 
a sudden inspiratory movement. 

ANIMAL HEAT 

The animal body possesses a temperature that is perceptible to the sense 
of touch and determinable by a thermometer. This temperature is the 
result of the liberation of heat which attends the chemic changes taking 
place in the tissues and organs of the living body and which underlie all 
manifestations of life. In consequence of this each animal acquires a cer- 
tain body-temperature. 

The normal temperature of the body in the adult, as shown by means of 
a delicate thermometer placed in the axila, ranges from 97.25^. to 
99.5°F., though the mean normal temperaturle is estimated by Wunderlich 
at 9 S.6 F. 

The temperature varies in different portions of the body however,. ac- 
cording to the extent to which oxidation takes place, being highest in the 
muscles, in the brain, blood, liver, etc. 

Variations in the Mean Temperature. — The conditions which produce 
variations in the normal temperature of the body are: age, period of the 
day, exercise, food and drink, climate, season, and disease. 

Age. — At birth the temperature of the infant is about i°F. above that of 
the adult, but in a few hours falls to 95.5^., to be followed in the course 
of twenty-four hours by a rise to the normal or a degree beyond. During 
childhood the temperature approaches that of the adult; in aged persons 
the temperature remains about the same, though they are not so capable 
of resisting the depressing effects of external cold as adults. A diurnal 
variation of the temperature occurs from i.8°F. to 3-7°F. (Jtirgensen) ; 
the maximum occurring late in the afternoon, from 4 to 9 P. M.; the mini- 
mum, early in the morning, from 1 to 7 A. M. 

Exercise. — The temperature is raised from i° to 2°F. during active con- 
tractions of the muscular masses, and is probably due to the increased 
activity of chemic changes; arise beyond this point being prevented by its 
diffusion to the surface, consequent on a more rapid circulation, radiation, 
more rapid breathing, etc. 

Food and Drink. — The ingestion of a hearty meal increases the tempera- 
ture but slightly; an absence of food, as in starvation, produces a marked 
decrease. Alcoholic drinks, in large amounts, in persons unaccustomed to 
their use, cause a depression of the temperature amounting to from i° to 
2°F. Tea causes a slight elevation. 



136 HUMAN PHYSIOLOGY 

External Temperature. — Long-continued exposure to cold, especially if 
the body is at rest, diminishes the temperature from i° to 2°F., while ex- 
posure to a great heat slightly increases it. 

Disease frequently causes a marked variation in the normal temperature 
of the body, which rises as high as io7°F. in typhoid fever and io5°F. in 
pneumonia; in cholera it falls as low as 8o°F. Death usually occurs when 
the heat remains high and persistent, from 106 to no°F.; the increase of 
heat in disease is due to excessive production rather than to diminished 
elimination. 

The Residual Heat of the Body. — As a preliminary to a consideration of 
heat-production and heat-dissipation, it is of interest to determine the 
actual quantity of heat expressed in Calories, that resides in the body at all 
times. This can be approximately determined from the chemic composi- 
tion and the temperature. A chemic analysis of the body shows that it 
consists of water 0.6, and of tissue 0.4. If the weight be assumed to be 70 
kilograms then 42 kilograms consist of water, and as the temperature is 
37°C, the 42 kilos of water will contain 42 X 37 or 1,554 kilogram calories; 
the remaining 28 kilograms consist of tissues, the specific heat of which is 
but 0.8 that of water, hence the 28 kilograms of tissue will contain 28 X 
0.8 calories; the equivalent of 22.4 kilograms of water. Since the 
temperature of the body is 37°C. the additional number of Calories will 
be 22.4 X 37 or 828, making a total of 2,382 Calories an amount of heat 
absolutely necessary to maintain the body-temperature at the physio- 
logical level. Notwithstanding the constant liberation of large amounts 
of heat each day, it is dissipated more or less rapidly in accordance with 
variations in temperature, character of clothing and a variety of other 
conditions, and so accurately is this done, that at the end of the twenty- 
four hours the body possesses its customary quantity of heat and its 
physiologic temperature. 

Heat Production. Thertnogenesis. — The immediate source of the body 
heat is to be found in the chemic changes that take place in all the tissues 
and organs of the body. 

Every contraction of a muscle, every act of secretion, each exhibition of 
nerve force, is accompanied by a change in the chemic composition of the 
tissues and an evolution of heat. The reduction of the disintegrated tis- 
sues to their simplest form by oxidation, and the combination of the oxygen 
of the inspired air with the carbon and hydrogen of the blood and tissues, 
results in the formation of carbonic acid and water and the liberation of a 
great amount of heat. 



ANIMAL HEAT 137 

Certain elements of the food, particularly the carbo-hydrates and the fats, 
undergo oxidation without taking part in the formation of the tissues, 
being transformed into carbon dioxid and water, and thus increase the 
sum of heat in the body. 

The total quantity of heat liberated each day may be approximately 
determined in at least two ways: (1) by determining experimentally the 
heat values of different food principles by direct oxidation; (2) by collect- 
ing and measuring with a suitable apparatus, a calorimeter, the heat 
evolved by the oxidation of the food within, and dissipated from, the 
body daily. 

By the direct oxidation of the food principles by means of a calorimeter, 
it has been determined, when they are burned to carbon dioxid and water, 
that 1 gram of protein yields approximately 5.6 Calories, 1 gram of fat 
9.353 C. and 1 gram of starch or sugar 4.1 16 C. In the body fat and sugar 
or starch are also burned to carbon dioxid and water. The protein, how- 
ever, is only in part burned to this extent; a part is changed to urea which 
when eliminated carries with it a portion of the original heat of the protein. 
In the body the protein yields 4.124 Calories. The total number of calor- 
ies liberated by the various diet scales (see page 53) can be readily de- 
termined by multiplying the quantities of the food principles by the fore- 
going factors. The diet scale of Vieordt, for example, yields the 
following : 

120 grams of protein 494.88 Calories 

90 grams of fat 841 .77 Calories 

330 grams of starch 1,358.28 Calories 

Total 2,694 • 93 Calories 

The total calories obtained from other diet scales would be as follows: 
1 Ranke's, 2,335; Voit's, 3,387; Moleschott's, 2,984; Atwater's, 3,331. These 
numbers indicate theoretically the total heat-production in the body 
daily. 

. The collection of the heat dissipated by a human being weighing 70 
kilos when placed in a suitable calorimeter reveals the fact that it 
amounts to from 2,300 to 3,000 Calories. 

The amount of heat liberated will naturally vary in accordance with a 
number of conditions but principally with variations as physiologic activ- 
ity, the quantity and quality of food and changes in the external tempera- 
ture. The chief factor that increases metabolism and hence heat produc- 
tion is a low external temperature. This in turn leads to increased 
physical activity and increased food consumption. The heat-production 
and elimination under such circumstances may reach 4,700 Calories a day. 



I 



138 HUMAN PHYSIOLOGY 

Heat Dissipation, Thermolysis. — From the preceding statements it is 
evident that the body is continually liberating heat in amounts daily far in 
excess of that necessary for the maintenance of the body-temperature. 
Should this heat be retained, the temperature of the body would be raised 
at the end of twenty-four hours an additional 18 or 2o°C. — a temperature 
far in excess of that compatible with the maintenance of physiologic proc- 
esses. That the body maybe kept at the mean temperature of 37°C. it is 
essential that the heat liberated be dissipated as fast as it is produced, or to 
state the problem in another way, the heat dissipated by the body must be 
replaced by an equal amount liberated, if equilibrium of temperature is to 
be maintained. The dissipation of the heat is accomplished in several 
ways: 

Assuming 2,500 Calories to be an average of heat liberared during a day 
of repose, the losses, in the ways stated in the foregoing paragraph, may be 
tabulated as follows: 

1. In Warming Food and Drink. — The average temperature of food and 
drink is about i2°C; the amount of both together is about 3 kilograms; 
the specific heat of food about 0.8 that of water. The absorption of body- 
heat, therefore, by the food amounts approximately to 3 X 0.8 X 25°C. = 
60 Calories = 2.8 per cent. With the removal of the end-products of the 
foods and drink from the body an equal amount of heat is carried out. 

2. In Warming the Inspired Air. — The average temperature of the air 
is i2°C; the amount of inspired air, about 15 kilograms; the specific 
heat of air, 0.26. The absorption of body -heat by the air until it attains 
the temperature of the body will, therefore, amount to 15 X 0.26 X 25 = 
97.5 Calories = $.8 per cent. The expired air removes from the body a 
corresponding amount. 

3. In the Evaporation of Water from the Lungs. — The quantity of 
water evaporated from the lungs may be estimated at 400 grams; as 
each gram requires for its evaporation 0.582 Calorie, the quantity of 
heat lost by this channel would be 400 X 0.582 = 232.8 Calories =9.4 
per cent. 

4. In the Evaporation of Water from the Skin. — The quantity of water 
evaporated from the skin may be estimated at 660 grams, causing a loss of 
heat by this channel of 660 X 0.582 = 384.1 Calories = 15.3 per cent. 

5. In Radiation and Conduction from the Skin. — The amount of heat lost 
by this process can be indirectly determined only by subtracting the total 
amount lost by the above-mentioned channels from the total amount pro- 
duced. Thus, 2,500 — 7,774.4 = 1,725.6 Calories = 69 per cent, would 
represent the average amount lost by radiation and conduction. 



EXCRETION 130 

Head dissipation is accomplished as shown in the foregoing tabulation 
mainly by radiation and conduction, 70 per cent., and by the evaporation 
of water from the lungs and skin, 25 per cent. The mechanism by which 
this dissipation is accomplished consists of the cutaneous and pulmonic 
blood-vessels and the sweat-glands which may be. therefore, regarded as 
thermolytic organs. The ratio of the heat loss between the evaporation 
of water and radiation will vary with the temperature, the season of the 
year, the character of the clothing, etc. 

Inasmuch as the mean temperature of the body remains practically con- 
stant, notwithstanding seasonal variations, it is apparent that heat-dissi- 
pation must be exactly balanced by heat-production. Should there be 
any want of correspondance between the two processes, there would arise 
either an increase or a decrease in the mean temperature. As both heat- 
production and heat-dissipation are variable factors, dependent on a vari- 
ety of internal and external conditions, their adjustment is accomplished 
by a complex self-regulating mechanism involving muscle, vascular, and 
secretor elements, coordinated by the nerve system. 

EXCRETION 

Excretion may be denned as the process by which the end-products of 
metabolism are removed from the body. As the retention of these end- 
products in the body would exert a deleterious influence on normal met- 
abolism, their prompt removal becomes essential to the maintenance of 
physiologic activity. The principal excretions of the body — urine, per- 
spiration, and bile — are, with the exception of those given off in the 
lungs, complex fluids in which are to be found in varying proportions the 
chief end-products of metabolism. 

The chief excretory organs, therefore, are the kidneys, skin and lungs. 

URINE 

Normal urine is of a pale yellow or amber color, perfectly transparent, 
with an aromatic odor, an acid reaction, a specific gravity of 1,020, and a 
temperature when first discharged of ioo°F. 

The color varies considerably in health, from a pale yellow to a brown 
hue, owing to the presence of the coloring-matter, urobilin or urochrome. 

The transparency is diminished by the presence of mucus, the calcium 
and magnesium phosphates, and the mixed urates. 

The reaction of the urine is acid, owing to the presence of acid phosphate 
of sodium. The degree of acidity, however, varies at different periods of 



140 HUMAN PHYSIOLOGY 

the day. Urine passed in the morning is strongly acid, while that passed 
during and after digestion, especially if the food is largely vegetable in 
character, is either neutral or alkaline. 

The specific gravity varies from 1,015 to 1,025. 

The quantity of urine excreted in twenty-four hours is between forty and 
fifty fluidounces, but ranges above and below this standard. 

The odor is characteristic, and caused by the presence of taurylic and 
phenylic acids, but is influenced by vegetable foods and other substances 
eliminated by the kidneys. 

The chemic composition of the urine is very complex and is determined 
partly by the metabolism of the constituents of the tissues and partly by 
the quantity and the quality of the food consumed and metabolized. 
Hence the composition will vary from day to day in accordance with the 
character of the food. An average composition is presented in the follow- 
ing table: 

The Chemic Composition of Urine 

Water 1500 . 00 c.c. 

Total solids 72 . 00 grams. 

Urea 33 . 18 grams. 

Uric acid (urates) 0.55 grams. 

Hippuric acid (hippurates) .40 grams. 

Kreatinin, xanthin, hypoxanthin, guanin, am- 1 

,. . 1 ■. > 11.21 grams, 

monium salts, pigment, etc I 

Inorganic salts; sodium and potassium sulphates, 1 
phosphates, and chlorids; magnesium and I 
calcium phosphates \ 27 . 00 grams. 

Organic salts: lactates, acetates, formates in small I 
amounts J 

Sugar a trace. 

Gases, nitrogen, and carbonic acid. 

The Total Solids. — It is frequently a matter of clinic interest to deter- 
mine the total amount of the solid constituents excreted in twenty-four 
hours. This may be attained approximately by multiplying the last two 
figures of the specific gravity by the coefficient, 2.33, of Haeser or Chris tison. 
The coefficient of Jones, 2.6, is believed by some observers to give more 
accurate results for conditions existing in this country. The result ex- 
presses the total solids in 1,000 parts: e.g., urine with a specific gravity of 
1.020 would contain 20 X 2.33, or 46.60 grams of solid matter per 1,000 c.c. 
If the amount passed in twenty-four hours be 1,500 c.c, the total solids 
would amount to 69.9 grams daily. 

Organic Constituents of Urine. — Urea is one of the most important of the 
organic constituents of the urine, and is present to the extent of from 2.5 



URINE 141 

to 3. 2. per cent. Urea is a colorless, neutral substance, crystallizing in four- 
sided prisms terminated by oblique surfaces. When crystallization is 
caused to take place rapidly, the crystals take the form of long, silky 
needles. Urea is soluble in water and alcohol; when subjected to pro- 
longed boiling, it is decomposed, giving rise to carbonate of ammonia. 
In the alkaline fermentation of urine, urea takes up two molecules of 
water with the production of carbonate of ammonia. 

The average amount of urea excreted daily has been estimated at about 
34 grams. As urea is one of the principal products of the breaking up of 
the protein compounds within the body, it is quite evident that the quan- 
tity produced and eliminated in twenty-four hours will be increased by any 
increase in the amount of protein food consumed, or by a rapid destruction 
of protein tissues, as is observed in various pathologic states, inanition, 
febrile conditions, fevers, etc. A farinaceous or vegetable diet will dimin- 
ish the urea production nearly one-half. 

Muscular exercise when the nutrition of the body is in a state of equili- 
brium does not seem to increase the quantity of urea. 

Seat of Formation and Antecedents of Urea. — As to the seat of urea 
formation there is some discussion. It is quite certain that urea pre-exists 
in the blood and is merely excreted by the kidneys. Experimental and 
pathologic facts point to the liver as the probable organ engaged in urea 
formation. Acute yellow atrophy of the liver, suppurative diseases of the 
liver, diminish almost entirely the production of urea, but increase the 
amount of the ammonium salts in the urine. The perfusion of the liver 
of a recently killed animal with a given amount of blood containing 
ammonium salts will be followed after the lapse of several hours by an 
amount of urea in the blood two or three times the normal quantity. 
These and other facts indicate that the chief seat of urea formation is to be 
found in the liver cells. 

The antecedents of urea, out of which the hepatic cells construct urea 
have, for chemic reasons as well as from the foregoing experimental re- 
sults, been shown to be the salts of ammonia, the carbonate, carbamate, and 
lactate. The source of the ammonia is probably in part the intestine, as 
this compound is one of the products of the hydrolysis and cleavage of the 
proteins during digestion. That this is the case is apparent from the fact 
that the Mood of the portal vein always contains more ammonia than the 
blood of any other region of the vascular apparatus. 

It has also been established that of the amino-acids circulating in the 
blood and tissues a certain number not needed for growth and tissue re- 
pair, undergo a cleavage into an XII 2 portion and a carbonaceous radicle. 



142 HUMAN PHYSIOLOGY 

The former is then converted into ammonia and subsequently into urea by 
the liver cells or perhaps the muscles and other tissues as well. The pro- 
tein cons titu tents of the tissues may in their katobolism likewise yield the 
NH2 element, which is also subsequently transformed into ammonia and 
urea. 

Uric acid is also a constant ingredient of the urine and is closely allied to 
urea. It is a nitrogen-holding compound, carrying out of the body a por- 
tion of the nitrogen. The amount eliminated daily varies from 0.5 to 1 
gram. Uric acid is a colorless crystal belonging to the rhombic system. 
It is insoluble in water, and if eliminated in excessive amounts, it is de- 
posited as a "brick-red" sediment in the urine. It is doubtful if uric acid 
exists in a free state, being combined for the most part with sodium and 
potassium bases forming urates. It is to be regarded as one of the terminal 
products of the decomposition of nucleic acid which in turn is derived from 
nuclein, a constituent of cell nuclei. 

Hippuric acid is found very generally in urine, though it is present only 
in small amounts. It is increased by a diet as asparagus, cranberries, 
plums, and by the administration of benzoic and cinnamic acids. It is 
probably formed in the kidney. 

Kreatinin. — This is a crystalline nitrogenous compound closely resem- 
bling kreatin, one of the constituents of muscle-tissue. The amount ex- 
creted daily is about 1 gram. The origin of kreatinin is not very clear. 
It is probably, however, that if kreatin is capable of transformation into 
kreatinin a certain portion is derived from the kreatin contained in the 
meat consumed as food. But as kreatinin is steadily excreted though in 
less amounts on a diet from which meat is excluded it is certain that this 
portion at least must have some other source containing nitrogen, and the 
inference is that it is one of the end-products of the protein metabolism 
that is taking places in tissues generally and more particularly in muscle- 
tissue. 

Xanthin, Hypoxanthin, Adenin, Guanin. — These compounds are also 
found in urine in small but variable amounts. They are nitrogenized 
compounds derived mainly from the metabolism of the nuclein bodies, and 
frequently spoken of as the purin bases. 

Indol, Skatol, Phenol, Cresol. — These compounds, products of the putre- 
factive changes in the derivatives of protein are present in variable amounts, 
associated with potassium sulphate (see page 160). These compounds are 
known as the ethereal sulphates. The extent to which they are present is 
taken as a measure of the extent of intestinal putrefaction. 



KIDNEYS 143 

Inorganic Salts. — Sodium and potassium phosphates, known as the 
alkaline phosphates, are found in both blood and urine. The total quan- 
tity excreted daily is about 4 grams. Calcium and magnesium phosphates, 
known as the early phosphates, are present to the extent of 1 gram. Though 
insoluble in water, they are held in solution in the urine by its acid con- 
stituents. If the urine be rendered alkaline, they are at once precipitated. 
Sodium and potassium sulphate are also present to the extent of about 2 
grams. The phosphoric and sulphuric acids which are combined with 
these bases enter the body for the most part in the foods, though there is 
evidence that they also arise by oxidation in consequence of the metabol- 
ism of proteins which contain phosphorus and sulphur. Sodium chlorid is 
the most abundant of the inorganic salts. It is derived mainly from the 
food. The amount excreted is about 15 grams in twenty-four hours. 

KIDNEYS 

The kidneys are the organs for the secretion of urine. They are situated 
in the lumbar region, one on each side of the vertebral column behind the 
peritoneum, and extend from the eleventh rib to the crest of the ilium; the 
anterior surface is convex, the posterior surface concave, the latter present- 
ing a deep notch, the hilus. 

The kidney is surrounded by thin, smooth membrane composed of white 
fibrous and yellow elastic tissue ; though it is attached ot the surface of the 
kidney by minute processes of connective tissue, it can be readily torn 
away. The substance of the kidney is dense, but friable. 

Upon making a longitudinal section of the kidney it will be observed that 
the hilus extends into the interior of the organ and expands to form a cavity 
known as the sinus. This cavity is occupied by the upper, dilated portion 
of the ureter, the interior of which forms the pelvis. The ureter subdivides 
into several portions, which ultimately give origin to a number of smaller 
tubes, termed calcyces, which receive the apices of the pyramids (Fig. 13). 

The parenchyma of the kidney consists of two portions — viz. : 

1. An internal or medullary portion, consisting of a series of pyramids or 
cones, some twelve or fifteen in number. They present a distinctly striated 
appearance, a condition due to the straight direction of the tubules and 
blood-vessels. 

2. An external or cortical portion, consisting of a delicate matrix contain- 
1 ing an immense number of tubules having a markedly convoluted appear- 
ance. Throughout its structure are found numerous small ovoid bodies, 
termed Malpighian corpuscles. 



144 



HUMAN PHYSIOLOGY 



The Uriniferous Tubules. — The kidney is a compound, tubular gland 
composed of microscopic tubules whose function it is to secrete from the 
blood those waste products which collectively constitute the urine. If the 




Fig. 13. — Longitudinal Section through the Kidney, the Pelvis of the 
Kidney, and a Number of Renal Calyces. — {Tyson, after Henle.) 
A. Branch of the renal artery. U. Ureter. C. Renal calyx. 1. Cortex. 1'. 
Medullary rays. 1". Labyrinth, or cortex proper. 2. Medulla. 2'. Papillary 
portion of medulla, or medulla proper. 2". Border layer of the medulla. 3, 3. 
Transverse section through the axes of the tubules of the border layer. 4. Fat of 
the renal sinus. 5, 5. Arterial branches. .* Transversely coursing medulla rays. 

apex of each pyramid be examined with a lens, it will present a number of 
small orifices, which are the beginning of the uriniferous tubules. From 
this point the tubules pass outward in a straight but somewhat divergen 
manner toward the cortex, giving off at acute angles a number of branche 



KIDNEYS 



145 



(Fig. 14) . From the apex to the base of the pyramids they are known as the 
tubules of Bellini. In the cortical portion of the kidney each tubule be- 
comes enlarged and twisted, and after pursuing an extremely convoluted 
course, turns backward into the medullary portion for some distance, 
forming the descending limb of Henle's loop: it then turns upon itself, 
forming the ascending limb of the loop, reenters the cortex, again expands, 
and finally terminates in a spheric enlargement known as M tiller's or 
Bowman's capsule. Within this capsule is contained a small tuft of blood- 
vessels, constituting the glomerulus, or Malp-ighian 
corpuscles. 

Structure of the Tubules. — Each tubule consists 
of a basement membrane lined by epithelium cells 
throughout its entire extent. The tubule and its 
contained epithelium vary in shape and size in dif- 
ferent parts of its course. The termination of the 
convoluted tube consists of a little sac or capsule, 
which is ovoid in shape and measures about Moo 
of an inch. This capsule is lined by a layer of 
flattened epithelial cells, which is also reflected 
over the surface of the glomerulus. During the 
periods of secretory activity the blood-vessels of 
the glomerulus become filled with blood, so that 
the cavity of the sac is almost obliterated; after 
secretory activity the blood-vessels contract and 
the sac-cavity becomes enlarged. In that portion 
of the tubule lying between the capsule and Henle's 
loop the epithelial cells are cuboid in shape; in 
Henle's loop they are flattened, while in the re- 
mainder of the tubule they are cuboid and 
columnar. 




Fig. 14. — Diagram- 
matic Exposition of 
the Method in which 

THE UrINIFEROUS TUBES 

Unite to Form Primi- 
tive Cones . — (Tyson, 
after Ludwig.) 



Blood-vessels of the Kidney. — The renal artery 
is of large size and enters the organ at the hilum; it 

divides into several large branches, which penetrate the substance 
of the kidney between the pyramids, at the base of which they form 
an anastomosing plexus, which completely surrounds them. From 
this plexus vessels follow the straight tubes toward the apex of the 
pyramids, while others enter the cortical portion and pass to the 
surface. In the course of the latter, small branches are given off, each of 
which soon divides and subdivides to form a ball of capillary vessels 
known as the glovierulus. These capillaries, however, do not anastomose, 



146 HUMAN PHYSIOLOGY 

but soon reunite to form an efferent vessel the caliber of which is less than 
that of the afferent artery. In consequence of this, there is a greater re- 
sistance to the outflow of blood than to the inflow, and, therefore, a higher 
blood-pressure in the glomerulus than in capillaries generally. The rela- 
tion of the glomerulus to the tubule is important from a physiologic point 
of view. As stated above, the glomerulus is received into and surrounded 
by the terminal expansion or capsule of the tubule. This capsule, formed 
by an invagination of the terminal portion of the tubule, consists of two 
walls, an outer one consisting of an extremely thin basement membrane, 
covered by flattened epithelial cells, and an inner one consisting appar- 
ently only of flattened epithelium which is reflected over and closely 
invests the glomerular blood-vessels. The blood is thus separated from the 
interior of the capsule by the epithelial wall of the capillary and the epithe- 
lium of the reflected wall of the capsule. After its exit from the capsule 
the efferent vessel of the glomerulus soon again divides and subdivides 
to form an elaborate capillary plexus which surrounds and closely invests 
the convoluted tubules. From this plexus as well as from the plexus which 
surrounds the straight tubules veins arise which pass toward and empty 
into veins at the base of the pyramids. The renal vein formed by the 
union of these latter veins emerges from the kidney at the hilum and finally 
empties into the vena cava inferior. 

The nerves to the kidney have their origin in the cells of small ganglia 
situated close to the semilunar ganglion. They pass to the kidney in the 
renal plexus and follow the course of the blood-vessels to their termina- 
tion. The small renal ganglia are in connection with the spinal cord by 
means of the small splanchnics. The nerve fibers have both vaso-con- 
strictor and vaso-dilatator functions. 

The Renal Duct. — The renal duct, the ureter, is a membranous tube, situ- 
ated behind the peritoneum about the diameter of a goose-quill, eighteen 
inches in length, and extends from the pelvis of the kidney to the base of 
the bladder, which it perforates in an oblique direction. It is composed 
of three coats: fibrous, muscle and mucous. 

Mechanism of Urine Formation. — Inasmuch as the kidney presents (1) 
an apparatus for nitration, the capsule with its enclosed glomerulus, and 
(2) an apparatus for secretion^ the tubule with its epithelium, it was 
originally inferred by Bowman that the elimination of the constituents 
of the urine from the blood is accomplished by the two-fold process of 
filtration and secretion; that the water and highly diffusible inorganic salts 
simply pass by diffusion through the walls of the blood-vessels of the 
glomerulus into the capsule of Miiller, while the urea and remaining organic 



KIDNEYS 147 

constituents are removed by true secretory action of the renal epithelium. 
Modern experimentation supports this view of renal action though subject 
to some modification. 

The progress of physiologic investigation has confirmed the view that the 
capsule and glomerulus form a passive apparatus for the passage of a fil- 
trate not merely of water and inorganic salts, but having the characteris- 
tics and composition of the blood plasma less its protein content. That 
the epithelium is not only a secretory apparatus removing organic constit- 
uents from the blood but is also an absorptive apparatus whereby water 
and inorganic salts may be returned to the blood when needed for nutritive 
purposes. The physical properties and chemic composition of urine are 
resultants of the cooperative action of these different factors. 

The Influence of Blood Pressure. — The filtration of urinary constit- 
uents from the glomerulus into Miiller's capsule depends largely upon the 
blood-pressure and the rapidity of blood flow in the renal artery and 
glomerulus. 

The pressure ' the blood in the glomeruli may be raised and the velocity 
increased: 

1. By an increase in blood-pressure generally. 

1. By an increase in the pressure of the renal artery alone. 

The first condition may be brought about by an increase in either the 
force or frequency of the heart's action or by a contraction of the arterioles 
of vascular areas in any or all parts of the body, excepting, of course, the 
renal vascular area. The second condition is brought about by a dilata- 
tion of the renal artery alone and possibly by a contraction of the efferent 
vessels of the glomeruli. 

The pressure of the blood in the glomeruli may be diminished and the 
velocity decreased: 

1. By a decrease in the blood-pressure generally. 

2. By a decrease in the pressure of the renal artery alone. 

The first condition is brought about by a decrease in either the force or 
frequency of the heart's action or by a dilatation of the arterioles of large 
vascular areas in any or all parts of the body. The second condition is 
brought about by contraction of the renal artery alone and possibly by a 
dilatation of the efferent vessels of the glomeruli. Coincident with the 
rise and fall of pressure in the glomerular capillaries there is a rise and fall 
in the rate of urinary flow. 

The Storage and Discharge of Urine. — Urination.— The urinary con- 
stituents, as soon as they are eliminated from the blood, pass into and 
through the uriniferous tubules and by them are discharged into the pelvis 



148 HUMAN PHYSIOLOGY 

of the kidney. They then enter the ureter by which they are conducted to 
the bladder. The immediate cause of this movement is undoubtedly a 
difference of pressure between the terminal portions of the tubules and the 
terminal portion of the ureter, aided by the peristaltic contraction of the 
muscle wall of the ureter. 

The Bladder. — The bladder is a reservoir for the reception and tempo- 
rary storage of the urine prior to its expulsion from the body; when fully 
distended it is ovoid in shape, and holds from 600 to 800 c.c. It is composed 
of four coats; serous, muscle (the fibers of which are arranged longitudinally 
and circularly), areolar, and mucous. The orifice of the bladder is con- 
trolled by the sphincter vesica, sl muscular band about Jlz of an inch in width. 
The muscle-fibers collectively constitute the detrusor urinae muscle. 

Nerve Mechanism of Urination. — When the urine has passed into the 
bladder, it is there retained by the sphincter vesicae muscle, kept in a state 
of tonic contraction by the action of a nerve center in the lumbar region 
of the spinal cord. This center can be inhibited and the sphincter relaxed, 
either reflexly, by impressions coming through sensory nerves from the 
mucous membrane of the bladder, or directly, by a voluntary impulse 
descending the spinal cord. When the desire to urinate is experienced, 
impressions made upon the vesical sensory nerves are carried to the centers 
governing the sphincter and detrusor urince muscles and to the brain. If 
now the act of urination is to take place, a voluntary impulse originating in 
the brain passes down the spinal cord and still further inhibits the sphincter 
vesicae center, with the effect of relaxing the muscle and of stimulating the 
center governing the detrusor muscle, with the effect of contracting the 
muscle and expelling the urine. If the act is to be suppressed, voluntary 
impulses inhibit the detrusor center and possibly stimulate the sphincter 
center. 

The genitospinal center controlling these movements is situated in that 
portion of the spinal cord corresponding to the origin of the third, fourth, 
and fifth sacral nerves. 

SKIN 

The Skin. — The skin, the external investment of the body, is a most 
complex and important structure, serving — 

1. As a protective covering. 

2. As an organ for tactile sensibility. 

3. As an organ for the elimination of excrementitious matters. 

The amount of skin investing the body of a man of average size is about 



SKIN 149 

twenty feet, and varies in thickness, in different situations, from }?i to Hoo 
of an inch. 

The skin consists of two principal layers — viz., a deeper portion, the 
cerium, and a superficial portion, the epidermis. 

The Corium. — The corium, or cutis vera, may be subdivided into a 
reticulated and a papillary layer. The former is composed of white fibrous 
tissue, non-striated muscle-fibers, and elastic tissue, interwoven in every 
direction, forming an areolar network, in the meshes of w r hich are deposited 
masses of fat, and a structureless, amorphous matter; the latter is formed 
mainly of club-shaped elevations or projections of the amorphous matter, 
constituting the papilla:; they are most abundant and well developed 
under the palms of the hands and upon the soles of the feet; they average 
1 ! 00 of an inch in length, and may be simple or compound; they are well 
supplied with nerves, blood-vessels, and lymphatics. 

The Epidermis. — The epidermis, or scarf skin, is an extravascular 
structure, a product of the true skin, and is composed of several layers of 
cells. It may be divided into two layers: the retc mucosum, or the Mal- 
pighian layer, and the horny or corneous. 

The former is closely adherent to the papillary layer of the true skin, and 
is composed of large nucleated cells, the lowest layer of which, the "prickle 
cells," contains pigment-granules, which give to the skin its varying tints 
in different individuals and in different races of men; the more superficial 
cells are large, colorless, and semi-transparent. The latter, the corneous 
layer, is composed of flattened cells, which, from their exposure to the at- 
mosphere, are hard and horny in texture; it varies in thickness from 3^ of 
an inch on the palms of the hands and soles of the feet to 3^ 00 of an inch in 
the external auditory canal. 

Appendages of the Skin. — Hairs are found in almost all portions of the 
body, and can be divided into — 

1. Long, soft hairs, on the head. 

2. Short, stiff hairs, along the edges of the eyelids and nostrils. 
Soft, downy hairs on the general cutaneous surface. 

They consist of a root and a shaft. The latter is oval in shape and about 
1 of an inch in diameter; it consists of fibrous tissue, covered externally 
by a layer of Imbricated cells, and internally by cells containing granular 
and pigment material. 

The root of the hair is embedded in the hair-follicle, formed by a tubular 

1 depression of the skin, extending nearly through to the subcutaneous tissue; 

its walls are formed by the layers of the corium, covered by epidermic cells. 

At the bottom of the follicle ii a papillary projection of amorphous matter, 



' 



150 HUMAN PHYSIOLOGY 

corresponding to a papilla of the true skin, containing blood-vessels and 
nerves, upon which the hair-root rests. The investments of the hair-roots 
are formed of epithelial cells, constituting the internal and esxtemal root- 
sheaths. 

The hair protects the head from the heat of the sun and from the cold, 
retains the heat of the body, prevents the entrance of foreign matter into 
the lungs, nose, ears, etc. The color is due to pigment matter. In old age 
the hair becomes more or less whitened. 

The Sebaceous Glands. — The sebaceous glands, embedded in the 
true skin, are simple and compound racemose glands, opening, by a 
common excretory duct, upon the surface of the epidermis or into the 
hair-follicle. They are found in all portions of the body, most abundantly 
in the face, and are formed by a delicate, structureless membrane, lined 
by flattened polyhedral cells. The sebaceous glands secrete a peculiar 
oily matter (the sebum), by which the skin is lubricated and the hairs 
are softened; it is quite abundant in the region of the nose and forehead, 
which often presents a greasy, glistening appearance; it consists of water, 
mineral salts, fatty globules, and epithelial cells. 

The vernix caseosa, which frequently covers the surface of the fetus at 
birth, consists of the residue of the sebaceous matter, containing epithelial 
cells and fatty matters; it seems to keep the skin soft and supple, and guards 
it from the effects of the long-continued action of the amniotic water. 

The Sudoriparous Glands. — The sudoriparous glands excrete the 
sweat. They consist of a massor coil of a tubular gland duct, situated 
in the derma and in the subcutaneous tissue, average H5 of an inch in 
diameter, and are surroundeed by a rich plexus of capillary blood-vessels. 
From this oil the duct passes in a straight direction up through the skin 
to the epidermis, where it makes a few spiral turns and opens obliquely 
upon the surface. The sweat-glands consist of a delicate homogeneous 
membrane lined by epithelial cells, whose function is to extract from the 
blood the elements existing in the perspiration. 

The glands are very abundant all over the cutaneous surface — as many 
as 3,528 to the square inch, according to Erasmus Wilson. 

The perspiration is an excrementitious fluid, clear, colorless, almost 
odorless, slightly acid in reaction, with a specific gravity of 1,003 to 1,004. 

The total quantity of perspiration excreted daily has been estimated 
at about two pounds, though the amount varies with the nature of the 
food and drink, exercise, external temperature, season, etc. 

The elimination of the sweat is not intermittent, but continuous: it takes 



EXTERNAL SECRETIONS 151 

place so gradually that as fast as it is formed it passes off by evaporation 
as insensible perspiration. Under exposure to great heat and exercise the 
evaporation is not sufficiently rapid, and it appears as sensible perspiration. 

Composition of Sweat 

Water 995 -573 

Urea 0.043 

Fatty matters 0.014 

Alkaline lactates 0.317 

Alkaline sudorates 1.562 

Inorganic salts 2 .491 



1,000.000 



Urea is a constant ingredient. 

Carbonic acid is also exhaled from the skin, the amount being about 
J2 00 of that from the lungs. 

Perspiration regulates the temperature and removes waste matters 
from the blood; it is so important that if elimination be prevented, death 
occurs in a short time. 

Influence of the Nerve System. — The secretion of sweat is regulated 
by the nerve system. Here, as in the secreting glands, the fluid is formed 
from material in the lymph-spaces surrounding the gland. Two sets of 
nerves are concerned — viz. : vasomotor, regulating the blood-supply; and 
secretor, stimulating the activities of the gland cells. Generally the two 
conditions, increased blood flow and increased glandular action, coexist. 
At times profuse clammy perspiration occurs, with diminished blood flow. 
Sweat centers are found in the spinal cord between the levels of the 
second thoracic and third lumbar nerves. The secretory fibers reach 
the perspiratory glands of the head and face through the cervical sympa- 
thetic; of the arms, through the thoracic sympathetic, ulnar, and radial 
nerves; of the leg, through the abdominal sympathetic and sciatic nerves. 
The course they pursue is similar to those of the vasomotor nerves with 

• which they are associated. 

The sweat-center is excited to action by mental emotions, increased 
temperature of blood circulating in the medulla and cord, increased venosity 

, of blood, many drugs, rise of external temperature, exercise, etc. 

EXTERNAL SECRETIONS 

Secretion is a term applied to a process by which complex fluids are 
fromed from the constituents of the lymph which are separated from the 



152 HUMAN PHYSIOLOGY 

blood-stream by the activities of the endothelial cells of the capillary wall, 
as the blood flows through the capillary blood-vessels. 

These separated materials may be utilized in several ways: 
i . For the repair of the tissues, for growth, for the liberation of energy. 
2. For the elaboration or production by specialized organs of a variety 
of complex fluids and specific materials, of widely different application. 
The fluids and specific materials thus formed are utilized for the most 
part to meet some special need of the body. All such fluids and mate- 
rials are termed secretions, and the organs by which they are formed are 
termed secretor organs. Secretions whether simple or complex may in 
a general way be divided into two groups, viz.: external and internal. 

External Secretions. — An external secretion may be defined as a more 
or less complex fluid formed by the secretor activities of epithelial cells of 
glands, which is discharged through well-defined ducts on the surfaces of 
the body, the skin or mucous membrane. The glands by which they are 
formed or secreted are known as glands of external secretion. 

Internal Secretions. — Internal secretions may be defined as more or 
less complex materials or agents formed by the activities of epithelial 
cells of organs, and which are discharged into, and distributed by the blood 
to organs and tissues near and remote, the activities of which they influence 
in varying ways and degrees. The glands by which they are formed or 
secreted are known as glands of internal secretion. 

Organs of External Secretions. — All organs belonging to this group 
consist primarily of a thin delicate homogeneous membrane, one side of 
which is covered with a layer of epithelial cells and the other side of which 
is closely invested by a network of capillary blood-vessels, lymph- vessels, 
and nerves. Though the epithelial cells have a general histologic resem- 
blance one to another, their physiologic function varies in different situa- 
tions, in accordance probably with their ultimate chemic structure, a fact 
which determines the difference in the character of the secretions. 

These organs may consist of a single layer of cells or a group of cells, and 
may be subdivided into — 

i. Secreting membranes. 

2. Secreting glands. 

The secreting membranes are the mucous membranes lining the 
gastro-intestinal, the pulmonary, and the genito-urinary tracts. The 
secreting glands are formed of the same histologic elements as the secreting 
membranes. They are formed by an involution of the mucous membrane 
or skin, the epithelium of whigh is variously modified structurally and 



EXTERNAL SECRETIONS 1 53 

functionally in the various situations in which they are formed. Like 
the membranes themselves, the glands are invested by capillary 
blood-vessels and supplied with lymph-vessels and nerves, of which the 
latter are in direct connection with the blood-vessels and epithelial cells. 
The interior of each gland is in communication with the free surface by 
one or more passageways known as ducts. 

These glands may be classified according as the involution is cylindrical 
or dilated as — 

i. Tubular. The tubular glands may be simple — e.g., sweat-glands, 
intestinal glands, fundus glands of the stomach; or compound — e.g., 
kidney, testicle, salivary, and lachrymal glands. 

2. Alveolar. The alveolar glands may also be simple — e.g., the seba- 
ceous glands, the ovarian follicles, meibomian glands; or compound, as 
the mammary glands and salivary glands. 

In the production of the secretion two essentially different processes 
are concerned: 

i. C hemic. — The formation and elaboration of the characteristic organic 
ingredients of the secreted fluids — e.g., pepsin, pancreatin — take place 
during the intervals of glandular activity, as a part of the general function 
of nutrition. They are formed by the cells lining the glands, and can 
often be seen in their interior with the aid of the microscope — e.g., bile in 
the liver-cells, fat in the cells of the mammary gland. 

2. Physical. — Consisting of a transudation of water and mineral salts 
from the blood into the interior of the gland. 

During the intervals of glandular activity only that amount of blood 
passes through the gland sufficient for proper nutrition; when the gland 
begins to secrete, under the influence of an appropriate stinulus, theblood- 

sels dilate and the quantity of blood becomes increased beyond that 
flowing to the gland during its repose. 

Under these conditions a transudation of water and salt takes place, 
washing out the characteristic ingredients, which are discharged by the 
gland ducts. The discharge of the secretion is intermittent; they arc 
retained in the glands until they receive the appropriate stimulus, when 
they pass into the larger ducts by the vis a tcrgo, and are then discharged 
by the contraction of the muscular walls of the ducts. 

The I glandular secretion is hastened by an increase in the 

blood-volume and pressure and retarded by a diminution. 

The Influence of the Nerve System.— The activity of every gland is 
controlled by nerve-centers situated in the central nerve system. These 



154 HUMAN PHYSIOLOGY 

centers may be excited to activity either by impressions made on the 
peripheral terminations of afferent nerves or by emotional states; or, 
possibly, by changes in the composition of the blood itself. As a 
rule, all normal secretion is a reflex act involving the usual mechanism, 
viz.: a receptive surface (skin, mucous membrane, or sense-organ), an 
afferent nerve, an emissive cell from which emerges an efferent nerve to 
be distributed to a responsive organ, the gland epithelium, though the 
secretion may in some instances be initiated by a psychic state. 

The structure of the glands of external secretion, the composition and 
physiologic actions of their secretions have in large part been considered 
in the foregoing chapter on Digestion. There remains, however, to be 
considered the mammary glands, the liver and the sebaceous glands. 



MAMMARY GLANDS 

The mammary glands, which secrete the milk, are two more or less 
hemispheric organs, situated in the human female on the anterior surface 
of the thorax. Though rudimentary in childhood, they gradually increase 
in size as the young female approaches puberty. 

The gland presents at its convexity a small prominence of skin (the nipple) 
which is surrounded by a circular area of pigmented skin (the areola). 
The gland proper is covered by a layer of adipose tissue anteriorly and is 
attached posteriorly to the pectoral muscles by a meshwork of fibrous 
tissue. During utero-gestation the mammary glands become larger, firmer, 
and more lobulated; the areola darkens and the veins become more promi- 
nent. At the period of lactation the gland is the seat of active histologic 
and physiologic changes, correlated with the production of milk. At 
the close of lactation the glands diminish in size, undergo involution, and 
gradually return to their original non-secreting condition. 

Structure of the Mammary Gland. — The mammary gland consists of 
an aggregation of some fifteen or twenty lobes, each of which is surrounded 
by a framework of fibrous tissue. The lobe is provided with an excretory 
duct, which, as it approaches the base of the nipple, expands to form a 
sinus or reservoir, beyond which it opens by a narrowed orifice on the 
surface of the nipple. On tracing the duct into a lobe, it is found to divide 
and subdivide, and finally to terminate in lobules or acini. Each acinus 
consists of a basement membrane, lined by low polyhedral cells. Exter- 
nally it is surrounded by connective tissue supporting blood-vessels, 
lymphatics and nerves. 



MILK 155 



MILK 



Milk is an opaque, bluish-white fluid, almost inodorous, of a sweet 
taste, an alkaline reaction, and a specific gravity of 1,025 to 1,040. When 
examined microscopically it is seen to consist of a clear fluid (the milk- 
plasma), holding in suspension an enormous number of small, highly 
refractive oil-globules, which measure, on an average, Ko 0,000 of an inch 
in diameter. Each globule is supposed by some observers to be surrounded 
by a thin, albuminous envelope, which enables it to maintain the discrete 
form. The quantity of milk secreted daily by the human female averages 
about two and a half pints. The milk of all mammalia consists of all 
the different classes of nutritive principles, though in varying proportions. 
The relative proportions in which these constituents exist are shown in 
the following table of analyses: 

The Composition of Milk 

Constituents ! Human I Cow 

Water 87.80 87.00 

Caseinogen 



} 



Lactalbumin ' 

Fat 3 .50 3.80 

Lactose 700 5.00 

Inorganic Salts ' . 20 0.50 




Caseinogen is the chief protein constituent of milk, and is held in solution 
by the presence of calcium phosphate. On the addition of acetic acid or 
of sodium chlorid up to the point of saturation, the caseinogen is precipi- 
tated as such, and may be collected by appropriate chemic methods. When 
taken into the stomach caseinogen is coagulated — that is, it is separated 
into casein or tyrein and a small quanity of a new soluble protein. The 
ferment which induces this change is known as rennin. The presence of 
calcium phosphate is necessary for this coagulation. 

Fat is present in the condition of a fine emulsion and is more or less 
solid at ordinary temperatures. It is a composition of olein, palmitin, 
and stearin, with a small quantity of butyrin and caproin. When milk 
is allowed to stand for some time the fat-globules rise to the surface and 
form a thick layer, known as cream. When subjected to the churning 
process, the fat globules run together and form a cohesive mass — the 
butter. 



156 HUMAN PHYSIOLOGY 

Lactose is the particular form of sugar characteristic of milk. It be- 
longs to the saccharose group and has the following composition: C12H22- 
On. In the presence of the Bacilus acidi lactici the lactose is in part 
reduced to lactic acid and carbon dioxid, the former of which will cause a 
precipitation of the caseinogen. It is the presence of lactic acid that 
imparts the sour taste to milk. 

Inorganic salts are always present and are chiefly those of potassium, 
sodium, calcium, amd magnesium, phosphates and chlorids. 

Iron is also present in small amounts possibly from 3 to 5 milligrams per 
1,000 c.c. Citric acid to the extent of 0.05 per cent, is also present. 

Mechanism of Secretion. — During the time of lactation the mammary 
gland exhibits periods of secretory activity which alternate with periods of 
rest. Coincidently with these periods, certain histologic changes take 
place in the secreting structures of the gland. At the close of a period of 
active secretion each acinus presents the following features: the epithelial 
cells are short, cubic, nucleated, and border a relatively wide lumen in 
which is to be found a variable quantity of non-discharged milk. After 
the gland has rested for some time, active metabolism again begins. The 
epithelial cells grow and elongate; the nucleus divides into two or three 
new nuclei, and at the same time the cell becomes constricted; the inner 
portion is detached and is discharged into the lumen. Coincidentally with 
these changes oil-globules makes their appearance in the cell protoplasm, 
some of which are discharged separately into the lumen, while others 
remain for a time associated with the detached cell. From these his- 
tologic changes it would appear that the caseinogen and the fat-globules 
are metabolic products of the cell protoplasm, and not derived directly 
from the blood. That lactose has a similar origin appears certain from 
the fact that it is formed independently of carbohydrate food. The water 
and inorganic salts are doubtless secreted by a mechanism similar to that 
of all other secreting glands. 

Colostrum. — Within a day or two after parturition the alveoli become 
filled with a fluid which in some respects resembles milk and which has 
been termed colostrum. This is a watery fluid containing disintegrated 
epithelial cells and fat-globules, as well as a colostrum corpuscles, which 
are probably leukocytes containing fine fat-globules. Colostrum is 
distinguished from milk in being richer in sugar and inorganic salts. It 
also differs from milk in undergoing coagulation by heat which is supposed 
to be due to the presence of a globulin. Its coagulation point is about 
72°C. It is said to possess constituents which act as a laxative to the 
young child, 



LIVER 157 



LIVER 



The liver is a highly vascular, conglomerate gland, appended to the 
alimentary canal. It is the largest gland in the body, weighing about four 
and one-half pounds; it is situated in the right hypochondriac region, and is 
retained in position by five ligaments, four of which are formed by dupli- 
catures of the peritoneal investment. 

The proper coat of the liver is a thin but firm fibrous membrane, closely 
adherent to the surface of the organ, which it penetrates at the transverse 
fissure, and follows the vessels in their ramifications through its substance, 
constituting Glissons capsule. 

Structure of the Liver. — The liver is made up of a large number of small 
bodies (the lobules), rounded or ovoid in shape, measuring 3^5 of an inch 
in diameter, separated by a space in which are situated blood-vessels, 
nerves, hepatic ducts, and lymphatics. 

The lobules are composed of cells, which, when examined microscopic- 
ally, exhibit a rounded or polygonal shape, and measure, on the average, 
Kooo of an inch in diameter; they possess one, and sometimes two, nuclei; 
they also contain globules of fat, pigment matter, and animal starch. The 
cells constitute the secreting structure of the liver, and are the true hepatic 
cells. 

The Blood-vessels. — The blood-vessels which enter the liver are: 

1. The portal vein, made up of the gastric^ splenic , and superior and 
inferior mesenteric veins. 

2. The hepatic artery, a branch of the celiac axis. 

Both the portal vein and the hepatic artery are invested by a sheath of 
areolar tissue. 

The vessels which leave the liver are the hepatic veins, originating in its 
interior, collecting the blood distributed by the portal vein and hepatic 
artery, and conducting it to the ascending vena cava. 

Distribution of Vessels. — The portal vein and the hepatic artery, upon en- 
tering the liver, penetrate its substance, divide into smaller and smaller 
branches, occupy the spaces between the lobules, completely surrounding 
and limiting them, and constitute the interlobular vessels. The hepatic 
artery, in its course, gives off branches to the walls of the portal vein and 
-on's capsule, and finally empties into the small branches of the portal 

in the interlobular spaces. 
The interlobular vessels form a rich plexus around the lobules, from which 
branches pass to neighboring lobules and enter their substance, where they 



158 HUMAN PHYSIOLOGY 

form a very fine network of capillary vessels, ramifying over the hepatic 
cells, in which the various functions of the liver are performed. The blood 
is then collected by small veins, converging toward the center of the 
lobule, to form the intralobular vein, which runs through its long axis 
and empties into the sublobular vein. The hepatic veins are formed by 
the union of the sublobular veins, and carry the blood to the ascending 
vena cava; their walls are thin and adherent to the substance of the hepatic 
tissue. 

Bile Capillaries and Hepatic Ducts. — The bile capillaries are narrow 
channels which penetrate the lobule in all directions and are generally 
found running along the sides of the cells. These channels, which are 
devoid of walls, receive from the cells some of the products of their seer e tor 
activity, and hence are comparable to the lumen of the alveoli of other 
secreting glands. At the periphery of the lobules the bile capillaries 
communicate with larger channels which are the beginnings of the hepatic 
or bile-ducts lying in the interlobular spaces. The interlobular bile-ducts 
possess a distinct wall lined by flattened epithelium. There is, however, 
no disinct line of demarcation between the cells of the interlobular ducts 
and the secreting cells of the liver proper, as the two blend insensibly, the 
one into the other. As the hepatic ducts increase in size they gradually 
acquire the structure characteristic of the main hepatic duct: viz., a 
mucous, a muscle, and a fibrous coat. Two ducts emerge from the liver 
which after a short course unite to form the main hepatic duct. The 
main hepatic emerges from the liver at the transverse fissure. At a dis- 
tance of about 5 centimeters it is joined by the cystic duct, the distal 
extremity of which expands into a pear-shaped reservoir — the gall-bladder 
in which a portion of the bile is temporarily stored. The duct formed by 
the union of the hepatic and cystic ducts — the common bile duct passes 
forward for a distance of about 7 centimeters and opens into the 
duodenum. 

Functions of the Liver. — The liver is a complex organ having a variety 
of relations to the general processes of the body. While its physiologic 
actions are not yet wholly understood, it may be said that it is engaged: 

1. In the secretion of bile. 

2. In the production of starch (glycogen) and sugar (glucose). 

3. In the formation of urea. 

4. In the conjugation of products of protein putrefaction. 

The Secretion of Bile. — The characteristic constituents of the bile do 
not preexist in the blood, but are formed in the interior of the liver cells of 
materials derived from the venous and arterial blood. The hepatic cells, 



LIVER 159 

absorbing these materials, elaborate them into bile-elements, and in so 
doing undergo histologic changes similar to those exhibited by other secre- 
tory glands. The bile once formed, it passes into the months of the bile 
capillaries, near the periphery of the lobules. Under the influence of the 
vis a tergo of the new-formed bile it flows from the smaller into the large 
bile-ducts, and finally empties into the intestine, or is regurgitated into the 
gall-bladder, where it is stored up until it is required for the digestive proc- 
ess in the small intestine. The study of the secretion of bile by means of 
biliary fistula? reveals the fact that the secretion is continuous and not 
intermittent; that the hepatic cells are constantly pouring bile into the 
ducts, which convey it into the gall-bladder. As this fluid is required 
only during intestinal digestion, it is only then that the walls of the gall- 
bladder contract and discharge it into the intestine. 

The flow of bile from the liver cells into the gall-bladder is accomplished 
by the inspiratory movements of the diaphragm, and by the contraction of 
the muscle-fibers of the biliary ducts, as well as the pressure of new-formed 
bile. Any obstacle to the outflow of bile into the intestine leads to an 
accumulation within the bile-ducts. The pressure within the ducts 
increasing beyond that of the blood within the capillaries, a reabsorption 
of biliary matters by the lymphatics takes place, giving rise to the phenom- 
ena of jaundice. 

The bile is both a secretion and an excretion; it contains new constituents, 
which are formed only in the substance of the liver, and are destined to play 
an important part ultimately in nutrition; it contains also waste ingre- 
dients, which are discharged into the intestinal canal and eliminated from 
the body. 

The Production of Glycogen and Sugar. — In addition to the preceding 
function, Bernard, in 1848, demonstrated the fact that the liver, during 
life, normally produces a substance analogous in its chemic composition 
to starch, which he termed glycogen; also that, when the liver is removed 
from the body, and its blood-vessels are thoroughly washed out, after a 
few hours sugar makes its appearance in abundance. The sugar can also 
be shown to exist in the blood of the hepatic vein as well as in a decoction 
of the liver substance by means of either Trommer's or Fehling's test, 
even when the blood of the portal vein does not contain a trace of sugar. 

Origin and Destination of Glycogen. — Glycogen appears to be formed 
in the liver cells, from materials derived from the food, whether the diet 
be animal or vegetable, though a larger percentage is formed when the 
animal is fed on starchy and saccharine than when fed on animal food. 
The dextrose, which is one of the products of digestion, is absorbed by the 



l6o HUMAN PHYSIOLOGY 

blood-vessels and carried directly into the liver; as it does not appear in 
the urine, as it would if injected at once into the general circulation, it is 
probable that it is detained in the liver, dehydrated, and stored up as 
glycogen. The change is shown by the following formula: 

C6H12O6 — H2O = C6H10O5. 

Dextrose. Water. Glycogen. 

The glycogen thus formed is stored up in the hepatic cells for the future 
requirements of the system. When sugar is needed for nutritive purposes, 
the glycogen is transformed into dextrose by the agency of a ferment. 

Glycogen, when obtained from the liver, is an amorphous, starch-like 
substance, of a white color, tasteless and colorless, and soluble in water; 
by boiling with dilute acids, or subjected to the action of an animal fer- 
ment, it is easily converted into dextrose. When an excess of sugar is 
generated by the liver out of the glycogen, dextrose can be found not only 
in the blood of the hepatic vein, but also in other portions of the vascular 
apparatus. 

The Formation of Urea. — The liver is now regarded by many physiolo- 
gists as the principal organ concerned in urea formation. 

The antecedent of the urea, the substances out of which the liver cells 
form urea, are for the most part the ammonium salts, the carbonate and 
carbamate, which are brought to the liver by the blood of the portal vein. 
These salts are formed largely in the intestinal wall out of the amino acids 
that result from the digestion of proteins. It is also very probable that 
they arise from the disintegration of amino-acids in other portions of 
the body. 

The Conjugation of Products of Protein Putrefaction. — One of the 

important functions of the liver is the conversion of toxic compounds, the 
products of the putrefaction of proteins, into non-toxic compounds. These 
compounds are formed in the intestine, are absorbed and carried by the 
blood of the portal vein to the liver. In their passage through the capil- 
laries of the liver they are conjugated for the most part with potassium 
sulphate by the action of the liver cells and thus deprived of their toxicity. 
Among the substances thus conjugated are indol, skatol, phenol, and cre- 
sol. After absorption indol and skatol are oxidized to indoxyl and ska- 
toxyl and then combined with potassium sulphate giving rise to potassium 
indoxyl sulphate and potassium skatoxyl sulphate. Phenol and cresol 
are apparently directly combined with potassium sulphate. All of these 
compounds then pass into the blood of the general circulation and finally 
are eliminated by the kidneys. Potassium indoxyl sulphate or indican 



INTERNAL SECRETIONS l6l 

is the source of the indigo-forming substance found in the urine. Other 
compounds are like-wise reduced in toxicity by the liver cells though the 
methods by which this is accomplished vary with the nature of the com- 
pound. The liver thus presents a chemic defense against the entrance of 
more or less toxic agents into the blood of the general circulation. 

INTERNAL SECRETIONS 

An internal secretion may be defined as a more or less complex material 
or agent, produced by the secretor activities of epithelial cells of organs 
and tissues, and which are discharged into the blood and distributed to 
organs more or less remote, the activities of which they influence in varying 
ways and degrees. Some increase, some inhibit physiologic processes 
while others stimulate growth and in different ways modify metabolism. 
The internal secretion in many, if not all instances belongs to a class of 
agents known as hormones, agents of known or unknown composition, 
characterized by a relatively simple chemic or molecular composition, an 
easy diffusibility across the walls of the capillary blood-vessels, a ready 
susceptibility to oxidation and a rapid elimination, as a result of which, 
their action does not continue indefinitely. 

Glands of Internal Secretion or Endocrinous Glands. — The glands con- 
sist mainly of epithelial cells in close relation to the walls of capillary 
blood-vessels and lymphatics, and in some instances, if not all, under the 
control of the central nerve system. By reason of the absence of ducts and 
their relation to blood-vessels they have also been termed ductless glands 
and vascular glands and inasmuch as the secretion is discharged internally 
(into the blood) they have been designated endocrinous glands. 

The glands which fall into this category are the thyroid, the parathy- 
roids, the adrenals, the hypophysis cerebri or . pituitary, the pancreas, 
the ovaries and testicles. 

Thyroid Gland. — The thyroid gland or body consists of two lobes 
situated on the lateral aspect of the upper part of the trachea. Each 
lobe is pyriform in shape, the base being directed downward and on a level 
with the fifth or sixth tracheal ring. The lobe is about 50 mm. in length, 
20 mm. in breadth, and 25 mm. in thickness. As a rule, the lobes are 
united by a narrow band or isthmus of the same tissue. In color the gland 
is reddish, and it is abundantly supplied with blood-vessels and lymphatics. 

Microscopic examination shows that the thyroid consists of an enormous 
number of closed sacs or vesicles, variable in size, the largesjt not measuring 
more than 0.1 mm. in diameter. Each sac is composed of a thin homo- 
11 



l62 HUMAN PHYSIOLOGY 

geneous membrane lined by cuboid epithelium. The interior of the sac in 
adult life contains a transparent viscid fluid, containing albumin and 
termed " colloid' ' substance. Externally, the sacs are surrounded by a 
plexus of capillary blood-vessels and lymphatics. The individual sacs 
are united and supported by connective tissue, which forms, in addition, a 
covering for the entire gland. The knowledge at present possessed as to 
the function of the thyroid gland, especially in mammals, is the outcome of 
a study of the effects which follow its arrest of development in the child, 
its degeneration in the adult, its extirpation in the human being as well 
as in animals. 

Congenital absence of the gland or its arrested development in early child- 
hood is followed by a defective physical and mental development charac- 
terized by the group of phenomena termed cretinism. 

Degenerative processes which arise in the thyroid in the adult give rise 
to a group of phenomena to which the term myxedema has been given. 
The most striking of these phenomena is a swollen condition of the skin, 
the result of a hyperplasia of the subcutaneous connective tissue of an 
embryonic type, rich in mucin. Partly in consequence of this change in 
the skin the face becomes broader, swollen and flattened with a loss of 
expression. With the progress of the degeneration, the mind becomes 
dull and clouded, the memory defective and finally the condition of 
idiocy may be established. 

Surgical removal of the thyroid when complete, for relief from symptoms 
due to grave pathologic changes, has been followed in human beings by 
symptoms similar, if not indentical with those of myxedema. To this 
condition the terms operative myxedema and cachexia strumipriva have 
been applied. Removal of the gland from animals is followed by the same 
symptoms and death in from two to three weeks. From these facts it is 
evident that the presence'of the thyroid is essential to the normal activity 
of the tissues generally. As to the manner in which it exerts its favorable 
influence, there is some difference of opinion. The view that the gland 
removes from the blood certain toxic bodies, rendering them innocuous, 
and thus preserving the body from a species of auto-intoxication, is grad- 
ually yielding to the more probable view that the epithelium is engaged in 
the secretion of a specific material, which finds its way into the blood or 
lymph and in some unknown way influences favorably tissue metabolism. 
This view of the function of the thyroid is supported by the fact that suc- 
cessful grafting of a portion of the thyroid beneath the skin or in the 
abdominal cavity will prevent the usual symptoms which follow thyroid- 
ectomy. The same result is obtained by the intravenous injection of 
thyroid juice or by the administration of the raw gland. The retention 



INTERNAL SECRETIONS 163 

of a small portion of the gland when it is removed by surgical means 
will prevent the occurrence of operative myedema. 

Hyperthyroidism, a condition characterized by vertigo, increased cardiac 
action, flushing, tremors, glycosuria, and in monkeys, exophthalmos and a 
widening of the palpebral fissure, may be developed by the administration 
of large doses of the gland extracts. From these facts the inference has 
been drawn from the clinical side that the symptoms comprised under the 
term exophthalmic goiter, viz.: rapid action of the heart, pulsation of the 
large arteries at the base of the neck, protrusion of the eyeballs and fine 
tremors of the hands, are due to an enlargement of the gland and a hyperse- 
cretion of the thyroid cells, a condition spoken of as hyperthyroidism. This 
inference has apparently been confirmed by the disappearance of the 
symptoms after the removal of a large portion of the gland, care being 
taken to leave a small portion sufficiently large, however, to produce the 
necessary amount of the internal secretion. 

The Thyroid Secretion. — The chemic features of the material secreted 
and obtained from the structures of the thyroid indicate that it is a complex 
protein containing iodin, which, under the influence of various reagents, 
undergoes cleavage, giving rise to a non-protein residue, which carries with 
it the iodin and phosphorus. The amount of iodin in the thyroid varies from 
0.33 to 1 milligram for each gram of tissue. To this compound the term 
thyro-iodin has been given. The administration of this compound pro- 
duces effects similar to those which follow the therapeutic administration 
of the fresh thyroid itself, viz. : a diminution of all myxedematous symp- 
toms. In normal states of the body, thyro-iodin influences very actively 
the general metabolism. It gives rise to a decomposition of fats and pro- 
teins and to a decline in body- weight. In large doses it may produce toxic 
symptoms, e. g., increased cardiac action, vertigo, and glycosuria. 

The Function of the Thyroid Gland. — The function or the physiologic 
action of the thyroid gland itself is to produce an internal secretion which 
after its entrance into the blood promotes favorably the metabolism of the 
neuro-muscular systems at least. The myxedema and the failure of the 
mental powers are attributed to the loss or degeneration of the gland and 
hence its internal secretion, and cretinism to the arrest of its development. 

The Parathyroids. — The parathyroids are small bodies, usually four in 
number, two on each side. They are divided into superior and inferior. 
The superior are situated internally and on the posterior surface in close 
relation to, and frequently imbedded in, the substance of the thyroid; the 
inferior are situated externally, sometimes in contact with, and at other 
times removed a variable distance from the thyroid. Microscopically the 



1 64 HUMAN PHYSIOLOGY 

parathyroids consist of thick cords of epithelial cells separated by septa of 
fine connective tissue and surrounded by capillary blood-vessels. Chemic 
analysis shows that they also contain iodin in combination with some 
organic compound. 

Effects of Parathyroid Removal. — The surgical removal of the para- 
thyroids is followed in the course of from two to five days by the death of 
the animal preceded in most instances by a series of symptoms which are 
embraced under the general term "tetany." These symptoms are fibril- 
lary contractions of muscles, tremors, spasmodic contractions and paraly- 
ses of groups of muscles and not infrequently convulsive seizures and coma. 
During the convulsion there is an acceleration of the heart-beat, and in- 
crease in the respiratory movements which frequently become dyspneic in 
character. There is also a loss of appetite, nausea, mucous vomiting, and 
diarrhea. Death may occur during a convulsion or from coma. (Morat 
and Doyon.) 

These results for the most part occur only when all the parathyroids are 
removed. It is asserted that even if one gland is retained the animal does 
not die. The above described symptoms may manifest themselves, how- 
ever, but they are slight in degree. 

The Hypophysis Cerebri. — This is a small body lodged in the sella turcica 
of the sphenoid bone. It consists of an anterior lobe, somewhat red in 
color, and a posterior lobe, yellowish-gray in color. The former is much 
the larger and partly embraces the latter. The anterior lobe is developed 
from an invagination of the epiblast of the mouth cavity, and consists of 
distinct gland tissue. The posterior lobe is an outgrowth from the brain 
and is connected with the infundibulum by a short stalk. It has been 
suggested that the term infundibular body be reserved for the posterior 
lobe. This distinction appears to be desirable, inasmuch as in their origin 
and structure they are separate and distinct bodies. 

Complete removal of the hypophysis cerebri, or the pituitary body, is 
always followed by a fatal result, preceded by symptoms not unlike those 
which follow removal of the thyroid: viz., unsteadiness of gait, muscular 
twitchings, lethargy, fall of blood pressure, lowering of the body tempera- 
ture, coma and death. 

Partial removal of the anterior lobe is much less fatal, though adult animals 
become adipose and degenerate sexually. Young animals remain under- 
sized and fail to develop sexual characteristics. Sexual infantilism per- 
sists. From these and^ similar facts it has been assumed that sexual in- 
fantilism is due to defective activity of the anterior lobe. Hyperactivity 



INTERNAL SECRETIONS 1 65 

of the anterior lobe in early life may lead to giantism and in the adult to 
acromegaly. 

Removal of the posterior lobe leads to an increased tolerance for and 
assimilation of sugar which eventually contributes to the formation and 
deposition of fat. On the contrary a hyperactivity of the posterior lobe 
leads to a diminished tolerance for sugar as shown by the appearance of 
hyperglycemia and glycosuria. The internal secretion of the posterior 
lobe is believed to be the hyaline granules which, streaming through the 
lobe, are discharged into the third ventricle. 

Intravenous injection of pituitary extracts or the pharmaceutical prepa- 
ration pituitrin is followed by a rise of blood pressure from a contraction of 
the arteriole muscles, and an inhibition of the heart. It also causes dila- 
tation of the renal vessels and stimulates specifically the renal cells to activ- 
ity, thus causing a marked diuresis. The extract also stimulates the non- 
striated muscles of the intestines, bladder, uterus, mammary glands, as 
well as the dilatator muscle of the iris. 

The Functions of the Pituitary or Hypophysis. — The functions of the 
pituitary body are related to the activities of the anterior and posterior 
lobes. The anterior lobe, through its internal secretion, stimulates the 
growth of the skeleton and associated tissues as apparently shown by the 
fact that an excess of secretion in early life leads to giantism and in adult 
life to acromegaly, while a deficiency of secretion leads to defective growth 
and the establishment of infantilism. The posterior lobe through its inter- 
nal secretion assists in the regulation of carbohydrate metabolism as shown 
by the fact that an excess of secretion lowers the assimilation capacity and 
thus develops glycosuria, while a deficiency of the secretion raises the as- 
similation capacity and leads to the production and accumulation of fat. 

Adrenal Bodies, or Suprarenal Capsules. — These are two flattened 
bodies, somewnat crescentic or triangular in shape, situated each upon the 
upper extremity of the corresponding kidney, and held in place by con- 
nective tissue. They measure about 40 mm. in height, 30 mm. in breadth, 
and from 6 to 8 mm. in thickness. The weight of each is about 4 gm. 

Histology. — The gland is covered externally by a fibrous tissue from 
which septa pass into the more central portions thus forming a framework 
for the support of blood-vessels and cells. 

A section of the gland shows just beneath the capsule an outer portion 
termed the eort> x and an inner portion termed the medulla. The cortex 
consists mainly of cuboid cells arranged in cylindric columns. The outer 
layers of cells are arranged in irregular masses forming what has been 



1 66 HUMAN PHYSIOLOGY 

called the zona glomerulosa. The medulla consists of uniting and inter- 
lacing cords of polyhedral cells, the cytoplasm of which contains granular 
matter and a distinct nucleus. When treated with chromic acid or chro- 
mium salts the cytoplasm stains a dull brown or yellow color. For this 
reason they are termed chromaffin cells. Similar cells are found in sym- 
pathetic ganglia. 

The gland receives blood from branches of the renal artery; it discharges 
its venous blood by way of the adrenal veins into the vena cava on the 
right side and the renal vein on the left side. The gland cells are excited to 
activity, the central nerve system through the splanchnics and their con- 
tinuations, branches from the semi-lunar ganglion. 

Destructive pathologic processes of the adrenals produce a profound dis- 
turbance of the nutrition first described by Addison and subsequently 
termed by Trousseau, Addison's disease, which is characterized by ex- 
treme muscular weakness and an incapacity for sustained muscle activity; 
a bronze-like discoloration of the skin and mucous membranes, disturbance 
of the digestive functions, indicated by indigestion, vomiting and diarrhea; 
a feeble action of the heart; a small feeble pulse; a low blood-pressure; a 
subnormal temperature and a feeble respiration. Death ensues from pa- 
ralysis of the respiratory muscles. 

Surgical removal of these bodies from various animals is invariably and 
in a short time followed by death, preceded by some of the symptoms 
characteristic of Addison's disease. Their development, however, is more 
acute. From the fact that animals so promptly die from extirpation of 
these bodies, and the further fact that the blood of such animals is toxic to 
those the subjects of recent extirpation, but not to normal animals, the 
conclusion was drawn that the function of the adrenal bodies was to re- 
move from the blood some toxic material the product of muscle metabo- 
lism. Its accumulation after extirpation gives rise to death through 
auto-intoxication. 

The intravenous injection of adrenal extracts is followed in a very short 
time by a marked rise in blood pressure and if the dose be large enough, by 
a cessation of the auricular beat, though the ventricular beat continues 
though with a slower rhythm. If the vagi are cut previous to the injec- 
tion or if the inhibition is removed by atropin, the rapidity and vigor of 
both auricles and ventricles are increased. Whether the inhibitory in- 
fluence is removed or not, there is a marked increase in the blood-pressure, 
though it is greater in the former than in the latter instance. This is 
attributed to a direct stimulation and contraction of the muscle-fibers of 
the arterioles themselves, and not to vaso-motor influences, as it occurs 
also after division of the cord and destruction of the bulb. The contrac- 



INTERNAL SECRETIONS 167 

tion of the arterioles is quite general, as shown by plethysmographic studies 
of the limbs, spleen, kidney, etc. The arterioles of the lungs and brain do 
not contract under its influence to the same extent as do the arterioles in 
other regions of the body, possibly for the reason that they are not so 
abundantly supplied with vaso-motor nerves. Applied locally to the 
mucous membranes, the adrenal extract produces contraction of the blood- 
vessels and pallor. 

The extract also diminishes the tonus of the muscle walls of the intestine 
and other viscera. Injection of the extract into the peritoneal cavity or 
into the veins causes hyperglycemia and glycosuria which may last for 
several hours. All these effects follow an injection of an extract of the 
medulla only. 

The internal secretion is represented by the alkaloid termed epincphrin 
or adrenalin. This alkaloid produces all the effects of the extracts. 

The nerve system influences the secretory activity of the adrenals. 
The major emotional disturbances increase by percentage of adrenalin in 
the blood which in turn leads to hyperglycemia and glycosuria. 

The Function of the Adrenal Gland. — The function of the adrenal gland, 
at least of the medullary portion, is to furnish an internal secretion which 
serves apparently to maintain that degree of frequency and force of the 
heart-beat and the contraction of the arteriole muscle necessary to main- 
tain the normal blood-pressure; to inhibit as occasion requires, the tonus of 
muscle walls of various viscera; to cause a mobilization of sugar in the 
blood when this is necessary, and to increase in some unexplained way the 
tonus and activity of the skeletal musculature. 

The Pancreas. — The pancreas though engaged in the production of an 
external secretion is yet, by reason of the specialized group of cells, the 
islands of Langerhans, to be regarded as an organ of an internal secretion 
as well. These islands it is generally believed are engaged in the secretion 
of an agent which after entering the blood is carried to the muscles where 
it activates or assists a glycolytic enzyme in promoting the oxidation of 
sugar; or it may inhibit normally the stimulating action of adrenalin on the 
liver cells and thus prevent an excessive output of sugar and the develop- 
ment of hyperglycemia. If the entire pancreas is extirpated and the ani- 
mal survive the operation, a glycosuria is soon established, followed by a 
series of symptoms resembling those observed in diabetes mellitus as it 
occurs in man, viz.: thirst, polyuria, loss of energy, decline in body- weight, 
etc., followed by death in a few weeks. Pathologic processes that involve 
a large portion of the pancreas likewise give rise to a similar series of 



l68 HUMAN PHYSIOLOGY 

phenomena, as ligation of the pancreatic duct, a procedure that leads to a 
destruction of all portions of the pancreas except the islands of Langerhans 
and without developing glycosuria has led to the inference that these 
islands are the agents engaged in the production of the internal secretion. 

The Testicles and Ovaries. — The testicles and ovaries are regarded at 
the present time as glands for the production of an internal secretion, as 
well as for the production of the characteristic reproductive elements. 

The removal of the testicles early in life and before the age of puberty 
leads to imperfect development of the vesiculae seminales and the prostate 
gland; in addition to these defects, there is a failure of development of the 
various and distinctly sexual characters peculiar to man and other animals 
as well. Sexual desire is wanting and the body frequently remains in the 
infantile state. Transplantation of the testicles, in cocks and in certain 
of the smaller mammals that have been castrated, has led to the develop- 
ment of secondary sexual characters which in no apparent way differed 
from those of control animals. 

The ovaries are also regarded as glands for the production of an inter- 
nal secretion, as well as for the production of characteristic reproductive 
elements. 

The removal of the ovaries of human beings early in life is an operation 
that is not often performed and hence it is difficult to state the results that 
might arise. Their removal in certain animals leads to an atrophy of the 
uterus, and in addition, to a failure of development of secondary sexual 
characters. Menstruation does not occur and the body does not reach 
maturity. The removal of the ovaries in adult life results in a cessation 
of menstruation, and the appearance of a variety of disorders of a bodily 
and mental character. Similar phenomena are frequently observed at the 
menopause, when the ovaries undergo degenerative changes. The admin- 
istration of extracts of the ovaries — oophorin tablets — is claimed to relieve 
some of the symptoms following the removal of the ovaries or occurring 
during the menopause. The transplantation of an ovary into the wall of 
the uterus or into the broad ligament after ovariotomy in women has, even 
after the lapse of two years, reestablished menstruation and awakened 
sexual desire. 

The Spleen. — Though a ductless gland it can hardly be said that the 
spleen is a gland of internal secretion inasmuch as no experimental pro- 
cedure supports such a view. Notwithstanding all the experiments 
which have been made to determine the functions of the spleen, it can 
not be said that any very definite results have been obtained. The fact 



THE ORGANS OF THE NERVE SYSTEM 1 69 

that the spleen can be removed from the body of an animal without 
appreciably interfering with the normal metabolism would indicate that 
its function is not very important. The chief changes observed after 
such a procedure are an enlargement of the lymphatic glands and an 
increase in the activity of the red marrow of the bones. The presence 
of large numbers of leukocytes in the splenic pulp and in the blood of 
the splenic vein suggested the idea that the spleen is engaged in the 
production of leukocytes, and to this extent contributes to the forma- 
tion of blood. The presence of disintegrated red blood-corpuscles has 
suggested the view that the spleen exerts a destructive action on func- 
tionally useless red corpuscles. These and other theories as to splenic 
functions have been offered by different observers, but all are lacking 
positive confirmation. 

Plethysmographic studies show, that the splenic volume increases and 
decreases in response to the rise and fall of blood pressure. In addition 
to these rhythmic variations the spleen steadily increases in volume for 
a period of five hours after digestion, and then steadily declines and 
returns to its former condition. 



THE CENTRAL AND PERIPHERAL ORGANS OF THE NERVE 

SYSTEM 

All the neurons that collectively constitute the nerve system are grouped 
into more or less completely organized masses termed organs which in 
accordance with their location may be divided into (1) central organs and 
(2) peripheral organs. 

The Central Organs. — The central organs of the nerve system are the 
encephalon and the spinal cord, lodged within the cavity of the cranium 
and the cavity of the spinal column respectively. The general shape of 
these two portions of the nerve system correspond with that of the cavities 
in which they are contained. The encephalon is broad and ovoid, the 
spinal cord is narrow and elongated. 

The encephalon is subdivided by deep fissures into four distinct, though 
closely related portions: viz., (1) the cerebrum, the large ovoid mass oc- 
cupying the entire upper part of the cranial cavity; (2) the cerebellum, 
the wedge-shaped portion placed beneath the posterior part of the cere- 
brum and lodged within the cerebellar fossae; (3) the isthmus of the 
encephalon, the more or less pyramidal-shaped portion connecting the 
cerebrum and cerebellum with each other and both with (4) the medulla 
oblongata. 



170 HUMAN PHYSIOLOGY 

The spinal cord is narrow and cylindric in shape. It occupies the spinal 
canal as far down as the second or third lumbar vertebra. 

The central organs of the nerve system are bilaterally symmetric, con- 
sisting of distinct halves united in the median line. The cerebrum is sub- 
divided by a deep fissure, running antero-posteriorly, into two ovoid 
masses termed cerebral hemispheres; the cerebellum is also partially sub- 
divided into hemispheres; the isthmus likewise presents in the median 
line a partial division into halves; the medulla oblongata and spinal cord 
are subdivided by an anterior or ventral and a posterior or dorsal fissure 
into halves, a right and a left. 

The Peripheral Organs. — The peripheral organs of the nerve system 
in anatomic and physiologic relation with the central organs, are the en- 
cephalic and the spinal nerves. 

The encephalic nerves, twelve in number on each side of the median line, 
are in anatomic relation with the base of the encephalon, and because of the 
fact that they pass through foramina in the walls of the cranium they are 
usually termed cranial nerves. 

The spinal nerves, thirty-one in number on each side, are in anatomic 
relation with the spinal cord, and because of the fact that they pass through 
foramina in the walls of the spinal column they are termed spinal nerves. 
As both cranial and spinal nerves are ultimately distributed to the struc- 
tures of the body — i.e., the general periphery — they collectively constitute 
the peripheral organs of the nerve system. 

The spinal nerves consist of two groups of nerve-fibers, a ventral and a 
dorsal group. Though closely intermingled in the common trunk of the 
spinal nerve they are distinctly separated near the spinal cord. Owing to 
their connection with the ventral and dorsal surfaces of the spinal cord 
they have been termed respectively the ventral and dorsal roots. Per- 
ipherally the ventral root fibers are distributed to skeletal muscles, glands, 
walls of blood-vessels and walls of various viscera: the dorsal root fibers 
are distributed to skin, mucous membranes, muscles, joints, etc. 

The relation of the ventral and dorsal roots of the spinal nerves to the 
spinal cord, the classification of their contained nerve fibers and their 
various functions have been considered in a previous section (see pages 42, 

44). 

The encephalic nerves also consist of afferent and efferent nerve 
fibers which pass for the most part to their destinations as separate and 
independent nerves. Their relation to the encephalon and the phenomena 
that follow their stimulation and. division and the functions attributed 
to them will be fully considered in a subsequent section. 



SPINAL CORD 171 

SPINAL CORD 

The spinal cord varies from 10 to 45 cm., in length; is 12 mm. in 
thickness, weighs 42 grams and extends from the atlas to the second 
lumbar vertebra, terminating in the filum terminate. It is cylindric in 
shape, and presents an enlargement in the lower cervical and lower dorsal 
regions, corresponding to the origin of the nerves which are distributed 
to the upper and lower extremities. The cord is divided into two lateral 
halves by the anterior and posterior fissures. It is composed of both 
white or fibrous and gray or vesicular matter, the former occupying the 
exterior of the cord, the latter the interior, where it is arranged in the 
form of two crescents, one in each lateral half, united by the central mass, 
the gray commissure; the white matter being united in front by the white 
commissure. 

Segmentation of the Spinal Cord. — For the elucidation of many prob- 
lems connected with the physiologic actions of the spinal cord, as well as of 
the symptoms which follow its pathologic impairment, it will be found 
helpful to consider the cord as consisting physiologically of a series of seg- 
ments placed one above the other, the number of segments correspond- 
ing to the number of spinal nerves. Each spinal segment would therefore 
comprise that portion of the cord to which is attached a pair of spinal 
nerves. The nerve-cells in each segment are in histologic and physio- 
logic relation with definite areas of the body, embracing muscles, blood- 
vessels, glands, skin, etc. 

If the exact distribution of the nerves of any segment were then known, 
its function could be readily stated. By virtue of this segmentation it 
becomes possible for each segment to act independently, or in cooperation 
with other segments, near or remote, with which they are associated by the 
intrinsic or associative cells and their axons; and the spinal cord itself is 
enabled to act as a unit. 

Structure of the Gray Matter. — The gray matter is arranged in the forms 
of two crescents, united by a commissural band, forming a figure resem- 
bling the letter H. Each crescent presents a ventral and a dorsal horn. 
The center of the commissure presents a canal which extends from the 
fourth ventricle downward to the filum terminale. The ventral horn 
is short and broad and does not extend to the surface. The dorsal horn 
is narrow and elongated and extends quite to the surface. It is covered 
and capped by the substantia gelatinosa. The gray matter consists pri- 
marily of a framework of fine connective tissue, supporting blood-vessels, 



172 HUMAN PHYSIOLOGY 

lymphatics, medullated and non-medullated nerve-fibers, and groups of 
nerve-cells. 

Nerve -Cells. — The nerve-cells are arranged in groups, which extend for 
some distance throughout the cord, forming columns more or less continu- 
ous. The first group is situated in the ventral horn, the cells of which 
are large, multipolar, and connected with the ventral roots of the spinal 
nerves, and are supposed to be motor in function. The second group is 
situated in the dorsal horn, the cells of which are spindle-shaped, and from 
their relation to the posterior roots are supposed to be sensory in function. 
The third group is situated in the lateral aspect of the gray matter, and is 
& quite separate and distinct, except 

\\ ? in the lumbar and cervical enlarge- 

f % • '•■ '■ & ments, where it blends with those 

X% - /^ lPllf9^r^\ f °^ ^he ven tral horn. A fourth 

/ e /MMlJfc\ e \ .--"*' group is situated at the inner base 
L ^JiUt'*': H B>V A of the dorsal horn; it begins about 

JjlP&ll WmPM /ifwfi t ^ ie seventn or eighth cervical nerve 

h, ^^»^™|Km^^» anc * extends downward to the second 

^^Hin^llfli'^Sl^r or tn ^ r d lumbar, being most prom- 

^^^^^^^Ld^L inent in the dorsal region. This 

\ \ j hw column is known as Clark's vesicular 

, / C column. 

Fig. is.-Scheme of the Conducing . The nerve cells ma y be divid f d 
Path in the Spinal Cord at the Third into three groups, viz. : intrinsic, 
Dorsal Nerve. — (Landois.) ~ , ~. 

The black part is the gray matter, v, efferent and afierent 0r "CeptlVC 
Ventral, hw, dorsal root, a, Direct, and . . r ,, «« ., ,, u - 

g,g, crossed, pyramidal tracts, b, Ven- Structure of the White Matter. 
tral column, ground bundle, c, Goll's _ T h e white matter surrounding 
column. d, Postero-external column. 

e,e, and f ,f, Mixed lateral paths. h,h, each lateral half of the cord is made 
Direct cerebellar tracts. r ri £ -i • -i 

up of nerve-fibers, some of which are 

continuations for the nerves which enter the cord, while others are derived 

from different sources. It is subdivided into — 

i. A ventral column, comprising that portion between the ventral roots 

and the ventral fissure, which is again subdivided into two parts: 

(a) An inner portion, bordering the ventral median fissure, the direct 
Pyramidal tract, or column of Turck; it contains motor fibers which do not 
decussate at the medulla, and which extend as far down as the middle 
of the dorsal region. 

(b) An outer portion, surrounding the ventral cornua, known as the ven- 
tral root zone, composed of short, longitudinal fibers which serve to connect 
different segments of the spinal cord (Fig. 15). 



SPINAL CORD 173 

2. A lateral column, the portion between the ventral and dorsal roots, 
which is divisible into — 

(a) The crossed pyramidal tract, occupying the dorsal portion of the 
lateral column, and containing all those fibers of the motor tract which 
have decussated at the medulla oblongata; it is composed of longitudi- 
nally running fibers, which are connected with the multipolar nerve-cells 
of the ventral cornua. 

(b) The direct cerebellar tract, situated upon the surface of the lateral 
column, consisting of longitudinal fibers which have their origin in the 
cells of Clark's column and which terminate in the cerebellum; it first 
appears in the lumbar region, and increases in thickness as it passes 
upward. 

(c) The ventral tract, lying just posterior to the ventral cornua. 

3. A dorsal column, the portion included between the dorsal roots and 
the dorsal fissure, also divisible into two portions: 

(a) An inner portion, the postero-internal column, or the column of 
Goll, bordering the dorsal median fissure, and 

(6) An external portion, the postero-external column, the column of 
Burdach, lying just behind the dorsal roots. 

The two portions of the dorsal column are composed of long and short 
commissural fibers, which connect different segments of the spinal cord. 

i FUNCTIONS OF THE SPINAL CORD 

The spinal cord, by virtue of its contained nerve-cells and nerve-fibers, 
may be regarded as composed of — 

1. Nerve centers, each of which has a special function; and — 

2. Conducting paths by which these centers are brought into relation 
with one another and with the cerebrum and its subordinate or under- 
lying parts. 

A. The Spinal Cord Segments as Local Nerve Centers 

The efferent cells of the spinal segments are the immediate sources of 
the nerve energy that excites in skeletal muscles, glands, vascular, and 
to some extent visceral muscles. 

The discharge of their energy may be caused: 

1. By variations in the composition of the blood or lymph by which they 
are surrounded or as the outcome of a reaction between the chemic 
constituents of the lymph on the one hand and the chemic constituents 
of the nerve-cell on the other hand. The excitation of the cell thus 
occasioned is termed automatic or autochthonic excitation. 



174 HUMAN PHYSIOLOGY 

2. By the arrival of nerve impulses, coming through afferent nerves 
from the general periphery, skin, mucous membrane, etc. 

3. By the arrival of nerve impulses descending the spinal cord from 
cells in the cortex of the cerebrum or subordinate regions. 

The excitation in the former instances is said to be reflex or peripheral 
in origin; in the latter instance direct or cerebral in origin. In the direct 
or cerebral excitations the skeletal muscle movements are due to volitional, 
the gland discharges and vascular and visceral muscle movements to 
emotional, phases of cerebral activity. 

Automatic Activity. — By this expression is meant a discharge of energy 
from the spinal nerve-cells occasioned by (a) a change in the chemic 
composition of the blood and lymph by which they are surrounded or prob- 
ably a reaction between the constituents of the lymph and the constitu- 
ents of the nerve-cell or (b) the developments within the cell of a stimulus, 
the so-called "inner stimulus," the outcome of metabolic activity. 

As illustrations of such activity may be mentioned: (a) the contraction 
of the abductor muscle of the larynx (the posterior crico- arytenoid) 
whereby the vocal membranes are separated and the glottis kept open 
under all circumstances except during the emission of a Vocal sound; (b) 
the contraction of the dilatator muscle of the iris; (c) the contraction of the 
anal and vesic sphincters; (d) the periodic contraction of the respiratory 
muscles; (e) the acceleration of the heart-beat; (/) the more or less con- 
tinuous contraction of the arteriole muscles whereby the blood-pressure 
is largely maintained. The nerve centers exciting these structures are 
inferred to be in a condition of continuous automatic activity though 
capable of modification by nerve impulses reflected to them from more 
or less distant sources. 

Reflex Activity. — It has already been stated that the nerve-cells in the 
spinal cord are capable of receiving and transforming afferent nerve 
impulses, the result of peripheral stimulation, into efferent nerve impulses, 
which are reflected outward to skeletal muscles, exciting contraction; 
to glands, provoking secretion; to blood-vessels, changing their caliber; 
and to organs, inhibiting or augmenting their activity. All such actions 
taking place through the spinal cord and medulla oblongata independently 
of sensation or volition are termed reflex actions. The mechanism 
involved in every reflex action consists of at least the following structures, 
viz.: 

1. A receptive surface; e.g., skin, mucuous membrane, sense organ, etc. 

2. An afferent fiber and cell. 

3.. An emissive cell, from which arises — 



SPINAL CORD 175 

4. An efferent nerve, distributed to — 

5. A responsive organ, as muscle, gland, blood-vessel, etc. 

If a stimulus of sufficient intensity be applied to the receptive surface, 
there will be developed in the terminals of the afferent nerve a series of 
nerve impulses which will be transmitted by the afferent nerve to, and re- 
ceived by, the dendrites of the emissive cell in the anterior horn of the 
gray matter. With the reception of these impulses there will be a disturb- 
ance in the equilibrium of the molecules of the cells, a liberation of 
energy, and a transmission of nerve impulses outward through the 
efferent nerve to the skeletal muscle, gland-epithelium, vascular or 
visceral muscle. 

In preceding sections many illustrations of reflex actions have been 
presented in connection with the consideration of the mechanism of 
mastication; the secretion of saliva; the muscle, glandular and vascular 
phenomena of gastric and intestinal digestion; the vascular and respiratory 
movements, the mechanism of micturition, etc. 

Special Reflex Movements. — Among the reflexes connected with the 
more superficial portions of the body there are some which are so fre- 
quently either increased or diminished in pathologic conditions of the 
spinal cord that their study affords valuable indications as to the seat 
and character of the lesions. They may be divided into: 

1. The skin or superficial reflexes. 

2. The tendon reflexes. 

3. The organ reflexes. 

The skin reflexes, characterized by contraction of underlying muscles, 
are induced by stimulation of the afferent nerve-endings of the skin — e.g., 
by pricking, pinching, scratching, etc. The following are the principal 
skin reflexes: 

1. Plantar reflex, consisting of contraction of the muscles of the foot, 
induced by stimulation of the sole of the foot; it takes place through the 
segments of the cord which give rise to the second and third sacral 
nerves. 

2. Gluteal reflex, consisting of contraction of the glutei muscles when the 
skin over the buttock is stimulated; it takes place through the segments 
giving origin to the fourth and fifth lumbar nerves. 

3. Cremasteric reflex, consisting of a contraction of the cremaster 
muscle and a retraction of the testicle toward the abdominal ring when the 
skin on the inner side of the thigh is stimulated; it takes place through the 
segments which give origin to the first and second lumbar nerves. 

4. Abdominal reflex, consisting of a contraction of the abdominal 



176 HUMAN PHYSIOLOGY 

muscles when the skin upon the side of the abdomen is gently scratched; 
it takes place through the spinal segments which give origin to the nerves 
from the eighth to the twelfth thoracic. 

5. Epigastric reflex, consisting of a slight muscular contraction in the 
neighborhood of the epigastrium when the skin between the fourth and 
sixth ribs is stimulated; it takes place through the segments of the cord 
which gives origin to the nerves from the fourth to the seventh thoracic 
inclusive. 

6. Scapular reflex, consisting of a contraction of the scapular muscles 
when the skin between the scapulae is stimulated; it takes place through 
the segments of the cord which gives rise to the nerves from the fifth 
cervical to the third thoracic inclusive. 

The skin or superficial reflexes, though variable, are generally present in 
health. They are increased or exaggerated when the gray matter of the 
cord is abnormally excited, as in tetanus, strychnin-poisoning, and 
disease of the lateral columns. 

The so-called "tendon reflexes" are characterized by a movement of 
certain parts of the body due to the contraction of certain muscles and 
are elicited by a sharp tap on their tendons. The fundamental condition 
for the production of the tendon reflex is a certain degree of tonus of the 
muscle, which is a true reflex, maintained by afferent nerve impulses 
developed in the muscle itself in consequence of its extension and hence 
compression of the end-organs, the muscle spindles, of the afferent nerves. 
When the muscle is passively extended, as it must be when the reflex 
is to be elicited, there is an exaltation of the tonus and an increase in the 
irritability. To this condition of the muscle due to passive tension, the 
term myotatic irritability has been given. If the muscle extension be 
now suddenly increased, as it is when the tendon is sharply tapped, the 
increased compression of the muscle spindles will develop additional 
afferent impulses which after transmission to the spinal cord will give 
rise to contraction of the corresponding muscle. The tendon reflexes 
are of much value in the diagnosis of certain lesions of the spinal cord. 

The following are the principal forms of the tendon reflexes : 

1. The Patellar tendon reflex or knee-jerk. This phenomenon is 
characterized by a quick extension of the leg from the knee downward, 
due to the contraction of the extensor muscles of the thigh, when the 
ligamentum patellae is struck between the patella and tibia. This reflex 
is best observed when the legs are freely hanging over the edge of a table. 
The patella reflex is generally present in health, being absent in only 
2 per cent.; it is greatly exaggerated in lateral sclerosis, in descending 



SPINAL CORD 177 

degeneration of the cord; it is absent in locomotor ataxia and in atrophic 
lesions of the anterior gray cornua. 

2. The tendo- Achilles reflex or ankle-jerk. This phenomenon is char- 
acterized by a flexion of the foot due to a contraction of the gastrocnemius 
muscle when the tendo-Achillis is struck. To elicit the contraction, 
the leg should be extended and the dorsum of the foot be pressed toward 
the leg so as to give to the gastrocnemius a slight degree of extension. If 
the tendon be now sharply struck a quick flexion of the foot is produced. 

3. Ankle clonus. — This phenomenon consists of a series of rhythmic 
contractions of the gastrocnemius muscle, varying in frequency from six 
to ten per second. To elicit this reflex, pressure is made upon the sole 
of the foot so as to extend the foot at the ankle suddenly and energetically, 
thus putting the tendo-Achillis and the gastrocnemius muscle on the 
stretch. The rhythmic movements thus produced continue so long as 
the tension within limits is maintained. Ankle clonus is never present 
in health, but is very marked in lateral sclerosis of the cord. 

4. The Toe reflex. — This phenomenon is characterized by a flexion 
of the foot, then of the leg and perhaps of the thigh when the great toe is 
strongly and suddenly flexed. It is present in those diseases of the 
spinal cord in which there is a pronounced patellar reflex. 

5. The Wrist and Elbow reflex. — These phenomena are characterized by 
an extension movement of the hand and arm when the tendons of the 
extensor muscles are sharply tapped. These reflexes are especially 
marked in primary lateral sclerosis of the cord in the upper portion. 

The organ reflexes, e.g., the activities of the genito-urinary organs, the 
stomach, intestines, gall-bladder, etc., which are induced by peripheral 
stimulation have been considered in connection with the physiologic 
action of these organs. The genito-urinary center is located in the lumbar 
region of the spinal cord. In diseased conditions of this region the genito- 
urinary reflexes are sometimes increased, at other times decreased or 
even abolished. 

Reflex Irritability. — The general irritability or quickness of response 
of the mechanism involved in a reflex act is a variable factor in different 
individuals and depends very largely on the degree of irritability of the 
intra-spinal mechanism. If this is exalted the entire duration of the 
reflex act is shorter; if it is lowered the duration is lengthened — e.g., 

The reflex excitability of the cord may be — 

1. Increased, by disease of the lateral columns, by the administration 
of strychnin, and, in frogs, by a separation of cord from the brain, the 
latter apparently exerting an inhibitor influence over the former and 
depressing its reflex activity. 

r " 



178 HUMAN PHYSIOLOGY 

2. Decreased, by destructive lesion of the cord — e.g., locomotor ataxia, 
atrophy of the anterior cornua — the administration of various drugs, and 
in the frog, by irritation of certain regions of the brain. When the cere- 
brum alone is removed and the optic lobes are stimulated, the time elapsing 
between the application of an irritant to a sensor surface and the resulting 
movement will be considerably prolonged, the optic lobes (Setschenow's 
center) apparently generating impulses which, descending the cord, 
retard its reflex movement. 

Special Nerve Centers in Spinal Cord. — Throughout the spinal cord 
there are a number of spinal nerve centers, capable of being excited reflexly 
and of producing complex coodinated movements. Though for the most 
part independent in action, they are subject to the controlling influences of 
the medulla and brain. 

1. Cilios pinal center, situated in the cord between the lower cervical 
and the third dorsal vertebra. It is connected with the dilatation of the 
pupil through fibers which emerge in this region and enter the cervical 
sympathetic. Stimulation of the cord in this locality causes dilatation 
of the pupil on the same side; destruction of the cord is followed by con- 
traction of the pupil. 

2. Genitos pinal center, situated in the lower part of the cord. This is a 
complex center, and comprises a series of subordinate centers for the control 
of the muscular movements involved in the acts of defecation, micturition, 
and ejaculation of semen, and of the movements of the uterus during 
parturition, etc. 

3. Vaso -motor centers, giving origin to both vaso-cons trie tor and vaso- 
dilatator fibers, which are distributed throughout the cord between the 
first thoracic and third lumbar nerves. 

Though acting reflexly, they are under the dominating influence of the 
center in the medulla. 

4. Sweat centers are also present in various parts of the cord. 

Direct or Cerebral Excitation. — The activity of the emissive cells of 
the spinal cord segments, due to the arrival of nerve impulses descending 
the spinal cord from the cerebrum, in consequence of psychic states of a 
volitional or of an affective or emotional character, will be considered in a 
subsequent paragraph entitled " encephalo-spinal conduction." 

B. The Spinal Cord Segments as Conductors 

The white matter of the spinal cord consists of nerve-fibers, the specific 
function of which is, 

1. To conduct nerve impulses from one segment of the cord to another. 



SPINAL CORD 179 

2. To conduct nerve impulses coming to the cord through afferent nerves, 
directly or indirectly to the encephalon. 

3. To conduct nerve impulses coming from the encephalon to the 
spinal cord segments. 

Intersegmental Conduction. — The spinal cord consists of a series of 
physiologic segments each of which has a special function and is associated 
through its related spinal nerve with a definite segment of the body. For 
the harmonious cooperation and coordination of all the spinal segments it 
is essential that they should be united by commissural or associative 
fibers. The cord thus becomes capable of complex and purposive reflex 
actions. 

Spino -encephalic, or Sensor Conduction. — The nerve impulses that 
are brought to the spinal cord by the afferent spinal nerve-fibers are 
transmitted by afferent paths in the cord for the most part to the cortex 
of the cerebrum where they are translated into conscious sensations. 
These paths are, therefore, termed sensor. The sensor tract passes through 
the cord, the medulla oblongata, the pons Varolii, the superior portion 
of the crus cerebri, the posterior third of the posterior limb of the internal 
capsule, to sensor perceptive areas in the cerebral cortex. The sensor 
pathway decussates at all levels of the spinal cord and medulla, and, there- 
fore, the sensibility of each side of the body is associated with the opposite 
side of the brain. 

The paths for the nerve impulses that give rise to different sensations 
have been variously located by different observers. The pathway for 
the impulses that give rise to the sensations of temperature has been 
located in the gray matter; the pathway for the impulses that give rise 
to the sensation of pain has been located in Gower's tract; the pathway 
for tactile impressions has been located in the posterior columns. 

Encephalo -spinal, or Motor Conduction. — The nerve-fibers which 
conduct volitional impulses from the brain downward to the ventral cornua 
arise in the motor centers of the cerebrum; they then pass downward 
through the corona radiata, the internal capsule, the inferior portions of the 
crura cerebri, the pons Varolii, to the medulla oblongata, where the 
tract of each side divides into two portions, viz. : 

1. The larger, containing ninety-one to ninety-seven per cent, of the 
abers, which decussates at the lower border of the medulla and passes 
down in the lateral column of the opposite side, and constitutes the crossed 
pyramidal trad. 

2. The smaller, containing three to nine per cent, of the fibers, does not 
it once decussate, but passes down the ventral column of the same side, 



I 



i8o 



HUMAN PHYSIOLOGY 



and constitutes the direct pyramidal tract, or the column of Turck. At a 
lower level this tract also decussates or crosses over to the opposite side 







Pig. 16.— Course of the Fibers for Voluntary Movement.— (Landois.) 
rJ&J^K* the i m0t ° r ne / V . eS of the .trunk; c, fibers of the facial nerve; B, corpus 
^^NfnA^Ttu^t^^iF^ 1 ^^ capsule; NJ > lenticular nucleus; P, 
re^ifn^J'hnlf. P°i the l*™ 1 '' Py l PJ r £ mids a .^ their discussion; 01, olive; Gr, 
Pyramidal tracts P° stenor root 5 AR > anterior root; x, crossed! and z, direct 

of the cord. The fibers of both the crossed and the direct pyramidal tracts 
come into relation by their terminal branches with the nerve-cells in the 
ventral cornua of the gray matter of the opposite side of the cord. 



THE MEDULLA OBLONGATA l8l 

Through this decussation each half of the cerebrum governs the mus- 
cle movements of the opposite side of the body. 

The fibers composing the crossed and the direct pyramidal tracts are, 
therefore, the channels by which the volitional nerve impulses are con- 
ducted from the motor area of the cortex to the multipolar cells in the 
ventral cornua of the gray matter of the spinal cord, and by them and their 
related nerves transmitted to the muscles. 



THE MEDULLA OBLONGATA 

The medulla oblongata is the expanded portion of the upper part of 
the spinal cord. It is pyramidal in form and measures 38 mm. in 
length, 20 mm. in breadth, 12 mm. in thickness, and is divided 
into two lateral halves by the anterior and posterior median fissures, 
which are continuous with those of the cord. Each half is again sub- 
divided by minor grooves into four columns — viz., ventral, pyramid, 
lateral and tract olivary body, restiform body, and dorsal pyramid. 

1. The ventral pyramid is composed partly of fibers continuous with 
those of the ventral column of the spinal cord, but mainly of fibers derived 
from the lateral tract of the opposite side by decussation. The united 
fibers then pass upward through the pons Varolii and crura cerebri, and 
for the most part terminate in the corpus striatum and cerebrum. 

2. The lateral tract is continuous with the lateral columns of the cord; 
its fibers in passing upward take three directions — viz., an external 
bundle joins the restiform body, and passes into the cerebellum; and inter- 
nal bundledecussates at the median line and joins the opposite ventral 
pyramid; a middle bundle ascends beneath the olivary body, behind the 
pons, to the cerebrum, as the fasciculus teres. The olivary body of each 
side is an oval mass, situated between the ventral pyramid and restiform 
body; it is composed of white matter externally and gray matter inter- 
nally, forming the corpus dentatum. 

3. The restiform body, continuous with the dorsal column of the cord, 
also receives fibers from the lateral column. As the restiform bodies pass 
upward they diverge and form a space (the fourth ventricle), the floor 
of which is formed by gray matter, and then turn backward and enter the 
cerebellum. 

4. The dorsal pyramid is a narrow white cord bordering the posterior 
median fissure; it is continued upward, in connection with the fasciculus 
teres, to the cerebrum. 



1 82 HUMAN PHYSIOLOGY 

THE PONS VAROLII 

The pons Varolii is united with the cerebrum above, the cerebellum 
behind, and the medulla oblongata below. It consists of transverse and 
longitudinal fibers, amidst which are irregularly scattered collections 
of gray or vesicular nervous matter. 

The transverse fibers unite the two lateral halves of the cerebellum. 

The longitudinal fibers are continuous — 

i. With the ventral pyramids of the medulla oblongata, which, inter- 
lacing with the deep layers of the transverse fibers, ascend to the crura 
cerebri, forming their superficial or fasciculated portions. 

2. With fibers derived from the olivary fasciculus, some of which pass 
to the tubercula quadrigemina, while others, uniting with fibers from the 
lateral and posterior columns of the medulla, ascend in the deep or 
posterior portions of the crura cerebri. 

The Gray Matter of the Medulla and Pons Varolii. — The gray matter 
of both the medulla oblongata and pons is continuous with that of the 
spinal cord. It is arranged however with much less regularity. It forms 
a thin layer just beneath the floor of the fourth ventricle. Special groups 
of nerve-cells are found in it, some of which give origin to different cranial 
nerves. 

Functions of the Medulla and Pons. — By virtue of the presence of nerve- 
cells and definite tracts of nerve fibers the structures may be regarded as 
consisting: 

i. Of nerve centers, each of which has a special function, and 

2. Of conducting paths, by which these centers are brought into rela- 
tion not only with one another but with the cerebrum, the cerebellum 
and the spinal cord. 

The efferent or emissive nerve cells are excited to action by the same 
factors that excite to action the motor or efferent cells of the spinal cord 
(see page 173), viz. : (a) by local causes, (b) by the arrival of nerve impulses 
reflected from the skin and (c) by nerve impulses descending from the cere- 
brum or subordinate regions. 

The groups of nerve cells may be regarded therefore as centers for auto- 
matic, reflex and volitional activities. Some of these centers are as 
follows : 

1. The cardiac centers, which exert (1) an accelerator action over the 
heart's pulsations through nerve fibers emerging from the spinal cord in 
the roots of the first and second dorsal nerves and reaching the heart 
tbrouglrthe sympathetic nerve; (2) an inhibitor or retarding action on the 



THE CRURA CEREBRI I 83 

rate of the heart-beat through efferent fibers in the trunk of the pneumo- 
gastric or vagus nerve. (See pages in and 112). 

2. A vaso-motor center, which regulates the caliber of the blood-vessels 
throughout the body in accordance with the needs of the organs and tissues 
for blood, through nerve-fibers passing by way of the spinal nerves to the 
walls of the blood-vessels. (See page 121). 

3. A respiratory center, which coordinates the muscles concerned in the 
production of the respiratory movements. (See page 133.) 

4. A mastication center, which excites to activity and coordinates the 
muscles of mastication. 

5. A deglutition center, which excites and coordinates the muscles con- 
cerned in the transference of the food from the mouth to the stomach. 

6. An articulation center, which coordinates the muscles necessary to the 
production of articulate speech. 

7. A diabetic center, stimulation of which gives rise to glycosuria. 

8. A salivary center, stimulation of which excites the discharge of saliva. 

The Medulla and Pons as Conductors. — The anterior pyramids of the 
medulla and their continuations through the more ventral portions of the 
pons, being portions of the general pyramidal tract, serve to conduct 
volitional efferent nerve impulses from higher portions of the brain to the 
spinal cord. Division of these pathways is at once followed by a loss of 
volitional control of the muscles below the section. 

The dorsal or tegmental portion, containing the fillet, serves to transmit 
afferent nerve impulses from the spinal cord to higher portions of the brain. 
Transverse division of one-half of the dorsal portion of the pons is followed 
by complete anesthesia of the opposite half of the body without any 
impairment of motion. 

The restiform bodies constitute a pathway between the spinal cord and 
the cerebellum. The transverse fibers of the pons associate opposite but 
corresponding portions of the cerebellar hemispheres. 



THE CRURA CEREBRI 

The crura cerebri are largely composed of the longitudinal 'fibers of the 
pons (anterior pyramids, fasciculi teretes); after emerging from the pons 
they increase in size, and become separated into two portions by a layer of 
dark-gray matter, the locus niger. 

The superficial portion, the crusla, composed in part of the anterior 
pyramids, constitutes the motor tract, which terminates, to some extent, 



1 84 HUMAN PHYSIOLOGY 

in the corpus striatum, but for the most part, in the cerebrum; the deep 
portion, made up of the fasciculi teretes and posterior pyramids and acces- 
sory fibers from the cerebellum, constitutes the sensor tract (the tegmentum). 
which terminates in the optic thalamus and cerebrum. 

The gray matter is situated beneath the aqueduct of Sylvius and contains 
groups of nerve-cells which give origin to the nerve-fibers composing the 
third or oculo-motor nerve. 

Function. — The crura are conductors of motor and sensor impulses; the 
gray matter assists in the coordination of the complicated movements of 
the eyeball and iris, through the motor oculi communis nerve. It also 
assists in the harmonization of the general muscular movements, as section 
of one cms gives rise to peculiar movements of rotation and somersaults 
forward and backward. 

THE CORPORA QUADRIGEMINA 

The corpora quadrigemina are four small grayish eminences situated 
beneath the posterior border of the corpus callosum and behind the third 
ventricle. They rest upon the lamina quadrigemina, which forms the 
roof of the aqueduct of Sylvius. The superior pair are the larger and are 
known as the superior quadrigeminal bodies, the superior colliculi or the 
pregemina; the inferior pair are the smaller and are known as the inferior 
quadrigeminal bodies, the inferior colliculi, or the post-gemina. 

External and somewhat inferior to the corpora quadrigemina are two 
small collections of gray matter the more external of which has been 
termed the external geniculate body or the pregeniculum, the more internal 
of which has been termed the internal geniculate body or the post-geniculum. 

Though these bodies are closely associated anatomically, they differ in 
origin, in their relations, and in their functions. 

The corpora quadrigemina show on microscopic examination that they 
are composed of nerve-cells and nerve-fibers, both of which are so intri- 
cate y arranged that it is difficult to trace their relation one to another and 
to adjoining structures. Some of the cells of the superior quadrigeminal 
body give origin to axons which pass downward and forward and terminate 
in brush-like expansions around the nuclei of origin of the oculo-motor, 
trochlear, and abducent nuclei; other cells are surrounded by the terminal 
branches of some of the fibers of the optic tract, though it is not probable 
that they are true visual fibers. Still other cells receive the terminal 
branches of axons the cells of origin of which are located in the occipital 
cortex of the cerebrum and which reach the superior quadrigeminal body 
by way of the optic radiation and internal capsule. 



CORPORA STRIATA AND OPTIC THALAMI 1 85 

The cells of the post-geminum give origin to axons which pass upward, 
forward, and outward, enter the internal capsule, and pass by way of the 
auditory tract to the cortex of the temporo-sphenoidal region of the cere- 
brum. Many of the fibers of the lateral fillet, a portion of the auditory 
tract, terminate in brush-like expansions around these same cells. There is 
thus established a connected pathway between the cochlea and the tem- 
poro-sphenoidal cortex. 

The external geniculate body is a terminal station for a portion of the 
fine visual fibers coming from the retina. From the cells of this body new 
axons arise which course forward and upward, enter the internal capsule 
and pass by way of the optic radiation to the cortex of the occipital region 
of the cerebrum. 

Functions. — From the anatomic relation of the superior quadrigeminal 
body (the pre-geminum) to the optic tract, the inference can be drawn that 
it is in some way essential to the performance of various reflex ocular move- 
ments and perhaps to the variations in size of the pupil. Experimental 
investigations and pathologic changes support the inference. 

Irritation of the pre-geminum in monkeys on one side is followed by 
diminution of the pupils first on the opposite side and then almost immedi- 
ately on the same side. The eyes at the same time are also widely opened 
and the eyeballs turned upward and to the opposite side. If the irritation 
be continued, motor reactions are exhibited in various parts of the body. 
Destruction of the pre-geminum in both monkeys and rabbits is followed 
by blindness, dilatation and immobility'of the pupils, with marked disturb- 
ance of equilibrium and locomotion (Ferrier). 

From the anatomic relation of the inferior quadrigeminal body (the 
post-geminum) to the lateral fillet, the basal tract for hearing, the inference 
may be drawn that it is in some way connected with the auditory process. 

Stimulation of the post-geminum gives rise to cries and various forms of 
vocalization. Pathologic states of this body are also attended by im- 
pairment of hearing and disorders of the equilibrium. 

From the foregoing facts it is probable that the corpora quadrigemina are 
associated with station and locomotion. Ferrier assumes that in these 
bodies " sensory impressions, retinal and others, are coordinated with 
adaptive motor reactions such as are involved in equilibration and 
locomotion." 

CORPORA STRIATA AND OPTIC THALAMI 

The corpora striata are two large ovoid collections of gray matter, 
situated at the base of the cerebrum, the larger portions of which are 



1 86 HUMAN PHYSIOLOGY 

embedded in the white matter, the smaller portions projecting into the 
anterior part of the lateral ventricle. Each striated body is divided by 
a narrow band of white matter into two portions — viz.: 

i. The caudate nucleus, the intraventricular portion, which is conic in 
shape, having its apex directed backward, as a narrow, tail-like process. 

2. The lenticular nucleus, embedded in the white matter, and for the 
most part external to the ventricle. On the outer side of the lenticular 
nucleus is found a narrow band of white matter, the external capsule; and 
between it and the convolutions of the island of Reil, a thin band of 
gray matter, the claustrum. 

The corpora striata are grayish in color, and when divided, present 
transverse striations, from the intermingling of white fibers and gray cells . 

Functions. — The functions of the cells composing the caudate and len- 
ticular nuclei are very obscure. It is stated by some experimenters that 
localized stimulation both of a physiologic and pathologic character is 
followed by a persistent rise of temperature varying from i° to 2.6°C. 

The optic thalami are two oblong masses situated in the ventricles pos- 
terior to the corpora striata, and resting upon the posterior portion of the 
crura cerebri. The internal surface, projecting into the lateral ventricles, 
is white, but the interior is grayish, from a commingling of both white 
fibers and gray cells. Separating the lenticular nucleus from the caudate 
nucleus and the optic thalamus is a band of white tissue, the internal 
capsule. 

Functions. — From the anatomic relation of the optic thalami to the 
general and special sense nerve-tracts, on the one hand, and to the cerebral 
cortex, on the other hand, it is assumed that they are connected with the 
production of sensations both general and special, and act as intermediaries 
between the peripheral sense-organs and the cortex. 

It is probable that in the thalamus visual, tactile, and labyrinthine im- 
pressions are received, coordinated, and reflected outward, with the result 
of producing various adaptive motor reactions connected with station and 
equilibrium. The thalamus is believed by some investigators to act also 
as an intermediary between emotional states and their expression in the 
muscles of the face, this power being lost in certain pathologic conditions. 
The power of regulating the temperature of the body has been also as- 
signed to the thalamus, as destruction of its anterior extremity is 
usually followed by a rise in temperature. 

The Internal Capsule. — The lenticular nucleus is enclosed on all sides 
by ascending and descending nerve-fibers. From the manner in which 



CORPORA STRIATA AND OPTIC THALAMI 



I8 7 



they surround and enclose the nucleus they have collectively been called 
the lenticular capsule. If a horizontal section of the cerebrum be made 
at a certain level so as to cut across the capsule and the enclosed nucleus 
an appearance similar to that shown in Fig. 17, will be presented. That 
portion of the capsule that lies between the caudate nucleus and the optic 
thalamus internally, and the lenticular nucleus externally is known as the 
internal portion of the lenticular capsule, or in its abbreviated form as the 




Fig. 17. — Horizontal Section of the Internal Capsule Showing the 
Position and Relation of the Motor Tracts for the Eye, Head, Tongue, 
Mouth, Shoulder (Shi.), Elbow (Elb.), Digits of Hand (Dig.), Abdomen (Aeb.), 
Hip, Knee (Kn.), Digits of Foot (Dig.). S. Sensor tract. O.T. Optic tract 
A. T. Auditory tract. 



internal capsule, while that portion between the external convex border of 
the lenticular nucleus and the claustrum is known as the external portion 
of the lenticular capsule or in its abbreviated form as the external capsule. 
At a given level the internal capsule may be said to consist of two seg- 
ments or limbs, an anterior, situated between the caudate nucleus and 
the anterior extremity of the lenticular nucleus, and a posterior, situated 
between the optic thalamus and the posterior extermity of the lenticular 
nucleus. The two segments unite at an obtuse angle, termed the knee, 



1 88 HUMAN PHYSIOLOGY 

which is directed toward the median line. The appearance which is 
presented at different levels varies however considerably. 

Functions. — The internal capsule has been shown by the results both 
of experiment and of pathologic processes to be, first, a pathway for the 
transmission of nerve impulses from the cerebral cortex to the pons, 
medulla, and spinal cord, which give rise to contraction of the muscles 
of the opposite side of the body; and, second, a pathway for the transmis- 
sion of nerve impulses coming from skin, mucous membrane, muscles, and 
special sense organs to the cortex, where they give rise to sensations general 
and special. It is, therefore, the common motor and sensor pathway. 
For the reason that it transmits both motor and sensor impulses, and for 
the further reason that it is frequently the seat of pathologic lesions which 
are followed by either a loss of motion or sensation or both, the internal 
capsule is one of the most interesting parts of the central nerve system. 
The motor tract is confined to the posterior one-third of the anterior 
segment and the anterior two-thirds of the posterior segment. The 
sensor tract is confined to the posterior one-third of the posterior segment, 
the extreme end of which also contains the optic and auditory tracts. 

The region of the anterior segment in front of the motor tract con- 
tains the fibers of the fronto-cerebellar tract, the function of which is 
unknown. 

The motor region contains fibers which descend from the cerebral 
cortex to nerve centers situated in the gray matter beneath the aqueduct 
of Sylvius, in the gray matter beneath the floor of the fourth ventricle, 
and in the anterior horns of the gray matter of the spinal cord, and 
in turn are connected by the cranial and spinal nerves with the muscles 
of the eye, head, face, trunk, and limbs. The positions occupied by 
these different tracts are shown in Fig. 17. 

From the fact that the internal capsule contains efferent or motor 
tracts, and afferent or sensor tracts, it is evident that a destructive lesion 
of the motor tract would be followed by a loss of motion; and of the 
sensor tract, by a loss of sensation, on the opposite side of the body. 

THE CEREBRUM 

The cerebrum is the largest portion of the encephalon, constituting 
about 85 per cent, of its total weight. In shape it is ovate, convex on its 
outer surface, narrow in front and broad behind. It is divided by a deep 
longitudinal cleft or fissure into halves, known as the cerebral hemispheres. 
The hemispheres are completely separated anteriorly and posteriorly by 
this fissure, but in their middle portions are united by a broad white band 



CEREBRUM 



189 




Fig. 18. — Diagram showing Fissures and Convolutions on the Lateral 
Aspect of the Left Hemi-cerebrum. 
F, Frontal. P, Parietal. T, Temporal and O, Occipital lobes. 5. Fissure of 
Sylvius. EPS. Epi-sylvian. PRS. Pre-sylvian. SBS. Sub-sylvian fissures. C. 
Central fissure or fissure of Rolando. PRC. Precentral fissure. SPFR. Super- 
frontal fissure. MEFR. Media-frontal fissure. SBFR. Sub-frontal fissure. PCPC. 
Post-central fissure. PTL. Parietal fissure. PAROC. Par-occipital. m EXOCC 
Ex-occipital fissures. SPTMP. Super-temporal fissure. MTMP. Medi-temporal 
fissure. 




Fig. ig. — Diagram Showing Fissures and Convolutions on the Mesal Aspect 
of the Left Hemi-cerebrum. 
c. Upper extremity of the central fissure. PARC. Para-central fissure. SPCL. 
Super-callosal fissure. CL. Callosal fissure. OC. Occipital fissurr. CLC. Calcarine 
fissure. CLT. Collateral fissure. 



190 HUMAN; PHYSIOLOGY 

of nerve-fibers, the corpus callosum. Each hemisphere or hemi-cerebrum 
is convex on its outer aspect, and corresponds in a general way with each 
side of the cavity of the skull; the inner or mesial surface is flat and forms 
the lateral boundary of the longitudinal fissure. 

The surface of each hemi-cerebrum presents a series of alternate inden- 
tations and elevations, known respectively as fissures or sulci, and convo- 
lutions or gyri. A knowledge of the situation and extent of the principal 
fissures and convolutions, as well as of their relation one to another, is 
essential to a clear understanding of many physiologic processes, clinical 
phenomena, and surgical procedures. A study of the accompanying 
Figs. 18, 19, diagrams, with the aid of the accompanying legends, will 
show the location and relations of the more important of these fissures 
and convolutions. 

Structure. — The gray matter of the cerebrum, about 3 mm. thick, is 
composed of five layers of nerve-cells : 

1. A superficial layer, containing a few small multipolar ganglion cells. 

2. Small ganglion cells, pyramidal in shape. 

3. A layer of large pyramidal ganglion cells with processes running 
off superiorly and laterally. 

4. The glandular formation containing nerve-cells. 

5. Spindle-shaped and branching nerve-cells of a moderate size. 

The white matter consists of medullated nerve-fibers which though 
intricately arranged may be divided into three systems, viz.: the 
commissural, the association and the projection. 

The commissural fibers unite corresponding areas of the cortex of each 
side. 

The association fibers unite neighboring as well as distant parts of the 
same hemisphere, and may, therefore, be divided into long and short 
fibers. 

The projection fibers unite certain areas of the cerebral cortex with the 
basal ganglia, the pons, the medulla oblongata, and the spinal cord. They 
are divided into: (1) afferent fibers which have their origin in the lower 
nerve centers at different levels and thence pass to the cortex; and (2) 
efferent fibers which have their origin in the cortex and thence pass to 
the lower nerve centers, terminating at different levels. 

The afferent fibers } the so-called sensor tract, which transmit nerve im- 
pulses coming from the general periphery and sense-organs, pass through 
the tegmentum as the mesial and lateral fillets, and thence to the cortex 
directly by way of the internal capsule, or indirectly through the interme- 
diation of the thalamic and subthalamic nuclei. The distribution of 



CEREBRUM 191 

these fibers to the various areas of the cortex will be stated in following 
paragraphs. 

The efferent fibers of the so-called motor tract, which transmit motor or 
volitional nerve impulses from the cortex to the pons, medulla, and spinal 
cord, emerge from the layer of pyramidal cells of the gray matter of the 
anterior or the pre-central convolution, the para-central lobule, and imme- 
diately adjacent areas. From this origin the axons descend through the 
white matter of the corona radiata, converging toward the internal cap- 
sule, into and through which they pass, occupying the anterior two-thirds 
of the posterior limb or segment. Beyond the capsule they continue to 
descend, occupying the middle three-fifths of the pes or crusta of the crus 
cerebri, the ventral portion of the pons, and eventually the anterior 
pyramid of the medulla oblongata. At this point the tract divides into 
two portions, viz.: 

1. A large portion, containing from ninety-one to ninety-seven per cent, 
of the fibers, which decussates at the lower border of the medulla and 
passes down the lateral column of the cord, constituting the crossed 
pyramidal tract. 

2. A small portion, containing from three to nine per cent, of the fibers, 
which does not decussate at the medulla, but passes down the inner side 
of the anterior column of the same side, constituting the direct pyramidal 
tract or column of Tiirck. 

After passing through the internal capsule, and as it descends through 
the crus, pons, and medulla, the cortico-efferent tract gives off a number 
of fibers which cross the median line and arborize around the nerve-cells 
of the gray matter beneath the aqueduct of Sylvius (the nuclei of origin 
of the third and fourth cranial nerves), and around the nerve-cells in the 
gray matter beneath the floor of the fourth ventricle (the nuclei of origin 
of the remainder of the motor cranial nerves). The remaining fibers 
go to form the crossed and direct pyramidal tracts and arborize around the 
cells in the anterior horn of the gray matter of the opposite side of the 
cord at successive levels. By this means the cortex is brought into 
anatomic and physiologic relation with the general musculature of the 
various cranial and spinal motor nerves. 

Functions. — The functions of the cerebrum comprehend, in man at 
least, all that pertains to sensation, cognition, feeling, and volition. All 
subjective experiences, which in their totality constitute mind, are 
dependent on and associated with the anatomic integrity and the phy- 
siologic activity of the cerebrum and its related sense-organs, the eye, 
ear, nose, tongue and skin. 



192 HUMAN PHYSIOLOGY 

From an examination of the anatomic development of the brain in 
different classes of animals, in different men and races of men, and from 
a study of the pathologic lesions and the results of experimental lesions 
of the brain, evidence has been obtained which reveals in a striking 
manner the intimate connection of the cerebrum and all phases of mental 
activity. 

i. Comparative anatomy shows that there is a general connection be- 
tween the size of the brain, its texture, the depth and number of convolu- 
tions, and the exhibition of mental power. Throughout the entire animal 
series, the increase in intelligence goes hand in hand with an increase in 
the development of the brain. In man there is an enormous increase 
in size over that of the highest animals, the anthropoids. The most 
cultivated races of men have the greatest cranial capacity; that of the 
educated European being about 116 cubic inches, that of the Australian 
being about 60 cubic inches, a difference of 56 cubic inches. Men dis- 
tinguished for great mental power usually have large and well- developed 
brains; that of Cuvier weighed 64 ounces; that of Abercrombie, 63 ounces; 
the average being about 48 to 50 ounces. Not only in size, but, above 
all, the texture of the brain must be taken into consideration. 

2. Pathology. — Any severe injury or disease disorganizing the hemispheres 
is at once attended by a disturbance or an entire suspension of mental 
activity. A blow on the hea/1, producing concussion, or undue pressure 
from cerebral hemorrhage, destroys consciousness; physical and chemic 
alterations in the gray matter have been shown to coexist with insanity, 
and with loss of memory, speech, etc. Congenital defects of organization 
from imperfect development are usually accompanied by a corresponding 
deficiency of intellectual power and of t\e higher instincts. Under these 
circumstances no great advance in mental development can be possible, 
and the intelligence remains of a low grade. In congenital idiocy not only 
is the brain of small size, but it is wanting in proper chemic composition, 
phosphorus, a characteristic ingredient of the nervous tissue, being largely 
diminished in amount. 

3. Experimentation upon the lower animals — e.g., the removal of the 
cerebral hemispheres, is attended by results similar to those observed in 
disease and injury. Removal of the cerebrum in pigeons produces com- 
plete abolition of intelligence, and destroys the capability of performing 
spontaneous movements. The pigeon remains in a condition of pro- 
sound stupor, which is not accompanied, however, by a loss of sensation 
or of the power of producing reflex or instinctive movements. The pigeon 
can be temporarily aroused by pinching the feet, loud noises, lights placed 
before the eyes, etc., but soon relapses into a state of quietude, being 



CEREBRUM 1 93 

unable to remember impressions and connect them with any train of 
ideas, the faculties of memory, reason, and judgment being completely 
abolished. 

4. Experimental interference with the blood supply to the cerebrum 
is followed by a diminished or complete cessation of its activities. 

The Localization of Functions in the Cerebrum. — By the term locali- 
zation of functions is meant the assignment of definite physiologic func- 
tions to definite anatomic areas of the cerebral cortex. From experiments 
made on the brains of animals, by the observation and association of 
clinical symptoms with pathologic lesions of the central nerve system, and 
from observation of the developmental stages of the embryonic brain, it 
has been established in recent years : 

1. That the general and special sense-organs of the body are associated 
through afferent nerve-tracts with definite though perhaps not sharply 
delimited areas of the cerebral cortex; and — 

2. That certain areas of the cortex are associated through efferent nerve- 
tracts with special groups of skeletal or voluntary muscles. 

Experimental excitation of a cortical area associated with a sense-organ 
is undoubtedly attended by the production of a sensation at least similar 
to that produced by peripheral excitation of the sense-organ itself; destruc- 
tion of the area is followed by an abolition of all the sensations associated 
with the sense-organ. For these reasons such areas are termed sensor. 

Experimental excitation of a cortical area associated with a group of 
skeletal muscles is attended by their contraction; destruction of the area is 
followed by their relaxation or paralysis. For these reasons such areas 
are termed motor. 

The Sensor Areas.- — The sensor areas which should theoretically be 
present in the cortex are primarily those which receive and translate into 
conscious sensations nerve impulses, developed by changes going on in the 
body itself; and secondarily those which receive and translate into con- 
scious sensations the nerve impulses developed in the special sense-organs 
by the impact of the external or objective world. In the former areas, are 
received the nerve impulses that come from the mucous membranes, 
muscles, joints, viscera, etc., and give rise to muscle, and visceral sensations. 
In the latter areas are received the nerve impulses that come from the sense- 
organs and give rise to cutaneous, gustatory, olfactory, auditory, and vis- 
ual sensations. A number of such sense areas may be predicated: e.g., 
areas of cutaneous and muscle sensibility, of gustatory, olfactory, and 
visual sensibility. 

I J 3 



194 



HUMAN PHYSIOLOGY 



The sensor areas occupy regions more or less widely separated, though 
they are associated one with the other by association fibers (Figs. 20, 21). 

1. The Cutaneous Area. — The area of cutaneous or tactile sensibility 
has been assigned to the post-central convolution on the lateral aspect, 
and to a portion of the super-frontal convolution and the lower half 
of the para-central lobule on the mesial aspect of the hemicerebrum. 

Destruction of the post-central convolution in monkeys by the electro- 
cautery and in man by disease has invariably led to a loss of sensibility, 
hemianesthesia, on the opposite side of the body without at the same time 




CONCRETE CONCEPT 



Fig. 20. — The Areas and Centers of the Lateral Aspect of the Human 
Hemicerebrum. — (C. K. Mills.) 



causing any loss of motion. The location and extent of the anesthesia 
corresponds, of course, with the location and extent of the lesion of 
the cortex. 

2. The Muscle Sense Area. — The area of muscle sensibility has been 
assigned to the region posterior to but adjoining the post-central con- 
volution and includes the anterior part of the super-parietal and sub- 
parietal convolution and perhaps the supra-marginal convolution on the 
mesial aspect of the hemicerebrum. 

The sensations which are evoked in response to the action of nerve 



CEREBRUM 



195 



impulses coming from tendons, muscles, etc., are those of passive position 
and the direction and duration of movements of parts of the body. Clinic 
observations and post-mortem findings indicate that lesions of this area are 
followed by a loss of the muscle sense. 

In addition to sensations of passive position and direction of movements, 
the sensations of temperature and deep pressure are also associated with 
the physiologic activities of this region of the parietal lobe. 

3. The Stereognostic Area. — The area of stereognostic perception. Stere- 
ognosis is the recognition of any object when placed in the hands, 




Fig. 21. — The Areas and Centers of the Mesial Aspect of the Human 
Hemicerebrum. — (C. K. Mills.) 

through its form, density, temperature, etc. . The area associated with 
stereognostic perception has been assigned to a portion of the super-parietal 
convolution and to the precuneus. 

A lesion of this area impairs or destroys the power of recognition of 
objects and establishes the condition of aster eo gnosis. 

4. The Gustatory Area. — The area for gustatory sensibility has been 
assigned to the sub-collateral convolution on the mesial aspect of the 
temporo-sphenoidal lobe. 

Disease processes involving this area give rise frequently to subjective 



196 HUMAN PHYSIOLOGY 

sensations of taste. Electric stimulation of this area in mammals 
causes movements of the lips, tongue, etc., which are usually associated 
with sensations of taste. 

5. The Olfactory Area. — The area for olfactory sensibility has been 
assigned to the anterior portion of the hippocampal convolution (the 
uncinate region)' and the anterior portion of the callosal convolution or 
gyrus fornicatus. Disease processes in this region give rise frequently 
to subjective sensations of odors which as a rule are of an unpleasant char* 
acter. Destruction of this area is followed by a loss of odor sensations. 

6. The Auditory Area.— The area of auditory sensibility has been as- 
signed to portions of the temporal lobe and may be divided into primary 
and secondary areas. 

The primary area is located in the posterior portion of the super-tem- 
poral convolution, and perhaps the posterior portion of the insula. 

The secondary areas are located one below and in advance and the other 
below and somewhat behind the primary area, both extending into the 
medi-temporal convolution. 

Unilateral destruction of the primary area is followed, however, only 
by a partial loss of hearing in the opposite ear, owing to partial decussation 
of the auditory nerve, which, however, may be recovered from, after a 
time, owing probably to a compensatory activity of the insular convolution. 
Bilateral destruction of this region is followed by complete deafness. 
The primary area is connected on the one hand with the basal auditory 
center (the internal geniculate body) by the auditory radiation and on 
the other hand with the secondary areas by association fibers. 

In the first of these areas there are cells in which the sounds of objects 
are registered (object hearing); in the second of these areas there are cells 
in which the sounds of words, letters, etc., are registered or memorized. 
If these areas are destroyed by disease the condition of object-deafness 
and word-deafness is established. If word-deafness alone exists, the pa- 
tient though able to recognize sounds is unable to understand spoken 
language and is in the condition of a man who is hearing a language of 
which he has not the slightest idea. The same holds true for the per- 
ception of sensations of sound produced by objects. 

In the temporal lobe there are other areas, some of which are more 
or less associated with the auditory nerve, such as intonation, equi- 
libratory and orientation areas. (For the afferent pathway to this area, 
see auditory nerve.) 

7. The Visual Area. — The area for visual sensibility has been assigned 
to portions of the occipital and parietal lobes and may be divided into 
primary and secondary areas. 



CEREBRUM I 97 

The primary area is located in a triangular-shaped area on the mesial 
surface of the occipital lobe, which includes the gray matter above and 
below the calcarine fissure (the cuneus and upper part of the lingual lobe), 
and to the gray matter of the first occipital convolution on the lateral 
aspect of the occipital lobe. Focal lesions of this area on one side are fol- 
lowed by lateral homonymous hemianopsia, which, however, does not 
involve, as a rule, the fovea or macula. It is, therefore, the area of 
homonymous half-retinal representation. The location of the area for 
macular or central vision is near the anterior extremity of the calcarine 
fissure. 

The secondary areas are located partly on the lateral aspect of the 
occipital lobe and partly in the supra-marginal and angular convolutions 
of the parietal lobe. The primary area is connected, on the one hand, with 
the basal visual centers (the external geniculate body and the thalamus) 
by the optic radiation and, on the other hand, with the secondary areas 
by association fibers. 

The area on the lateral aspect of the occipital lobe is rather extensive, 
reaching down as far as the third and fourth occipital convolutions. Clin- 
ical evidence indicates that the cortex of this entire area is associated 
with the registration or memorization of the visual sensations and per- 
ceptions of objects, though it may be subdivided into smaller areas foi 
the registration of the visual sensations of different groups of objects 
such as geometric and architectonic forms, of persons, places and natural 
objects. Diseased processes in this region of the brain may result in 
the condition known as object blindness. The area on the lateral aspect 
of the parietal lobe (the supra-marginal and angular convolutions) are 
associated with the memorization of the visual sensations and perceptions 
of words, letters, numbers, and perhaps objects. If the visual word area 
is destroyed by disease, word blindness is established, and the patient 
is unable to understand written or printed language because of his in- 
bility to revive memory images of words. Letter and number blindness 
may or may not be present according to the extent of the lesion. (For 
the afferent pathway to these areas, see optic nerve). 

The Motor Areas. — The motor areas which should theoretically be 
present in the cortex are those which in consequence of the discharge 
of nerve impulses excite contraction of special groups of muscles and which 
from their coordinate and purposive character, are conventionally termed 
volitional. Five such general motor areas may be predicated: e.g., one 
for the muscles of the head and eyes, one for the muscles of the face and 
associated organs, and others for the muscles of the arm, leg, and trunk. 



198 HUMAN PHYSIOLOGY 

They are usually .designated as head and eye, face, arm, leg, and trunk 
motor areas. 

The existence and anatomic location of these areas in the cortex of 
animals have been determined by the employment of two methods of 
experimentation: viz., stimulation and destruction or extirpation; the 
first by means of the rapidly repeated induced electric currents, the second 
by the electric cautery and the knife. 

If the stimulation of a given area is attended by phenomena which 
indicate that the animal is experiencing sensation, and its destruction 
by a loss of this capability or the loss of a special sense, it is assumed that 
the area is sensor in function — is an area of special sense. If the stimu- 
lation or excitation of any given area is followed by contraction, and its 
destruction by paralysis of muscles, it is assumed that the area is motor 
in function — is an area of motion. 

The motor areas are assigned to the precentral convolution, the contig- 
uous^portions of the base of the medi- and subf rontal convolutions and the 
paracentral lobule. 

The main motor areas are as follows: 

1. The Head and Eye Area. — This area has been assigned to the con- 
tiguous portions of the medi- and subfrontal convolutions just anterior 
to the precentral convolution. It is subdivided into smaller areas which 
initiate and govern the movements of the head and eyeballs. Stimulation 
of this area, in the chimpanzee at least, produces turning of the head to 
the opposite side with conjugate deviation of the eyes to that side. 

2. The Face Area. — This area has been assigned to the lower portion of 
the precentral convolution and extends from below upward to about the 
level of the genu of the central fissure. This rather large area may be 
subdivided into (a) an upper portion including about one-third of the 
whole and (b) a lower portion including the remaining two-thirds. In 
both the upper and lower portions, there are groups of nerve-cells which 
excite to action, the muscles imparting movements to (a) the angle of 
the mouth, the eyelids and jaws and (b) the movements of the vocal 
bands or cords, the opening and closure of the mouth, the protrusion 
and retraction of the tongue. All of these movements have their areas 
of representation in the face area. 

3. The Arm Area. — This area has been assigned to the precentral con- 
volutions just above and contiguous to the face area which it exceeds some- 
what in extent. It is the largest of all the subdivisions of the general area. 
It may be divided into at least five smaller areas, the cells of which excite 
to action the muscles imparting movements to the thumb, the fingers, the 
wrist, the elbow and the shoulder. 



CEREBRUM I 99 

4. The Trunk Area. — This area has been assigned to the precentral 
convolution just superior to the arm area and is rather limited in extent. 
Horsley located a portion of this area on the mesial and lateral edges of 
the hemisphere in front of the leg area. The nerve-cells of this area when 
electrically stimulated excite to action the muscles, impart movements to 
the spinal column, such as arching rotation, etc. 

5. The Leg Area. — This area has been assigned to the extreme upper por- 
tion of the precentral convolution and to the adjoining mesial surface, the 
upper portion of the paracentral lobule. The area on the lateral aspect 
of the cerebrum may be subdivided into at least four smaller areas con- 
taining groups of nerve-cells which excite to action the muscles imparting 
movements to the toes, ankle, knee and hip. Evidence from the clinical 
side has demonstrated the fact that a localized irritative lesion of any one of 
these areas gives rise to convulsive movements of the muscles of the oppo- 
site side of the body, similar in character to those resulting from electric 
simulation of the corresponding areas of the monkey and ape brains. 
Destruction of these areas from the growth of tumors, softening, etc., is 
followed by paralysis of the muscles. Electric stimulation of these areas 
of the human brain for the purpose of localizing obscure irritative lesions 
prior to surgical procedures on the brain gives rise to similar convulsive 
movements. 

The Motor Speech Area. — By this term is meant an area of the cortex, 
the function of which is to arrange language for outward expression; for 
the use of the executive centers concerned with speech, e.g., the laryngeal, 
lingual and facial centers located at the foot of the precentral convolution. 
This area, i.e., the motor speech area, has been assigned to the posterior 
part of the subfrontal convolution (Broca's convolution) on the left side in 
those who are right-handed and on the right side in those who are congeni- 
tally left-handed, and in the anterior part of the insular or perhaps the pre- 
insular convolutions. Unipolar faradic stimulation of this area fails to call 
forth any motor response; its destruction by disease, however, is followed 
by a more or less extensive loss of the faculty of articulate speech or the 
faculty of expressing ideas with words, a condition usually spoken of as 
motor aphasia or aphemia. This area and the area at the foot of the pre- 
central convolution are united by association fibers. 

The Motor Writing Area. — By this term is meant an area of the cortex, 
the function of which is to arrange language for outward projection; for 
the use of the executive centers concerned with writing, viz.: the arm cen- 
ters located in the middle portion of the precentral convolution. This 
area, i.e., the motor writing area, has been assigned to the posterior half or 
third of the medi-frontal convolution. Unipolar faradic stimulation of this 



200 



HUMAN PHYSIOLOGY 



area fails to call forth any motor response; its destruction by disease, 
however, is followed by an inability to express ideas by writing, a condi- 
tion usually spoken of as agraphia. This area and the general arm center in 
the precentral convolution are united by association fibers. 

THE CEREBELLUM 

The cerebellum is situated in the inferior fossae of the occipital bone, 
beneath the posterior lobes of the cerebrum. It attains its maximum 




Fig. 22. — View of Cerebellum in Section and of Fourth Ventricle, with 
the Neighboring Parts. — {From Sappey.) 
i. Median groove forth ventricle, ending below in the calamus scriptorius, with 
the longitudinal eminences formed by the fasciculi teretes, one on each side. 2. The 
same groove, at the place where the white streaks of the auditory nerve emerge 
from it to cross the floor of the ventricle. m 3. Inferior peduncle of the cerebellum, 
formed by the restiform body. 4. Posterior pyramid; above this is the calamus 
scriptorius. 5, 5. Superior peduncle of cerebellum, or processus e cerebello ad 
testes. 6, 6. Fillet to the side of the crura cerebri. 7, 7. Lateral grooves of the 
crura cerebri. 8. Corpora quadrigemina. — (After Hirschfeld and Leveille.) 

weight, which is about one hundred and forty grams, between the 
twenty-fifth and fortieth years. 

It is composed of two lateral hemispheres and a central elongated lobe, the 
vermiform process; the two hemispheres are connected with each other by 
the fibers of the middle peduncle, forming the superficial portion of the 
pons Varolii. The cerebellum is brought into connection with the medulla 
oblongata and spinal cord through the prolongation of the restiform bodies; 



CEREBELLUM 201 

with the cerebrum, by fibers passing upward beneath the corpora quadri- 
gemina and the optic thalami, and then forming part of the diverging 
cerebral fibers. 

Structure. — It is composed of both white and gray matter, the former 
being internal, the latter external, and is convoluted, for economy of 
space. 

The white matter consists of a central stem, the interior of which is a 
dentated capsule of gray matter, the corpus dentalum. From the external 
surface of the stem of white matter processes are given off, forming the 
lamina, see Fig. 22, which are covered with gray matter. 

The gray matter is convoluted and covers externally the laminated proc- 
esses; a vertical section through the gray matter reveals the following 
structures: 

1. A delicate connective-tissue layer, just beneath the pia mater, con- 
taining rounded corpuscles, and with branching fibers passing toward the 
external surface. 

2. The cells of Purkinje, forming a layer of large, nucleated, branched 
nerve-cells sending off processes to the external layer. 

3. A granular layer of small but numerous corpuscles. 

4. A nerve-fiber layer y formed by a portion of the white matter. 

Properties and Functions. Irritation of the cerebellum is not followed 
by any evidences either of pain or convulsive movements; it is, therefore, 
insensible and inexcitable. 

Coordination of Movements. — Removal of the superficial portions of the 
cerebellum in pigeons produces feebleness and want of harmony in the 
muscular movements; as successive slices are removed, the movements 
become more irregular, and the pigeon becomes restless; when the last por- 
tions are removed, all power of flying, walking, standing, etc., is entirely 
gone, and the equilibrium cannot be maintained, the power of coordinating 
muscular movements being wholly lost. The same results have been 
obtained by operating on all classes of animals. 

The following symptoms were noticed by Wagner, after removing the 
whole or a large part of the cerebellum: 

1. A tendency on the part of the animal to throw itself on one side, and 
to extend the legs as far as possible. 

2. Torsion of the head on the neck. 

3. Trembling of the muscles of the body, which was general. 

4. Vomiting and occasional liquid evacuations. 



202 HUMAN PHYSIOLOGY 

Forced Movements. — Division of one cms cerebelli causes the animal to 
fall on one side and roll rapidly on its longitudinal axis. According to 
Schiff, if the peduncle be divided from behind, the animal falls on the same 
side as the injury; if the section be made in front , the animal turns to the 
opposite side. 

Disease of the cerebellum partially corroborates the result of experi- 
ments; in many cases symptoms of unsteadiness of gait, from a want oj 
coordination, have been noticed. 

Comparative anatomy reveals a remarkable correspondence between the 
development of the cerebellum and the increase in complexity of muscular 
actions. It attains a much greater development, relatively to the rest of 
the brain, in those animals whose movements are very complex and varied 
in character, such as the kangaroo, shark, and swallow. 

THE AUTONOMIC NERVE SYSTEM 

The Autonomic nerves comprise all the nerves that are distributed to 
the non-striated muscle-fibers in the walls of the blood-vessels, in the walls 
of the viscera and to the epithelium of all glands. These nerves consist of 
two consecutively arranged neurons, the first of which arises in the central 
nerve system and is termed preganglionic; the second of which arises in 
ganglionic cells and is, therefore, termed postganglionic. Inasmuch as the 
central nerve-cells giving origin to the preganglionic fibers are independent 
of volitional control (in marked contrast to the nerve-cells giving origin to 
the fibers for skeletal muscles) this system of nerves possesses a certain 
degree of autonomy, and has been termed the autonomic system. 
Though independent of volitional control they are influenced in the way 
of increased or decreased activity, by psychic states of an affective or 
emotional character. Their activity, however, is mainly excited by nerve 
, impulses transmitted to them from the surfaces of the body. 

The Physiologic Anatomy of the Autonomic Nerve System. — In a con- 
sideration of the essential facts of the physiologic anatomy of this system 
it will be convenient to consider first, the sympathetic ganglia and the dis- 
tribution of their postganglionic fibers. 

The Sympathetic Ganglia. — These ganglia may be divided into 3 groups, 
viz. : the vertebral, the prevertebral and the peripheral. From each of these 
groups non-medullated nerve-fibers pass in different directions. The 
vertebral ganglia give off fibers which under the name gray rami communi- 
cantes pass backward into the trunks of the spinal nerves and are distrib- 
uted to the blood-vessels of the skin of the trunk, arms and legs, as well 
as to the epithelium of the sweat-glands of the corresponding regions. 



AUTONOMIC NERVE SYSTEM 203 

The fibers of the upper cervical ganglia pass directly to the blood-vessels 
and sweat-glands of the head and face, while others pass directly to 
viscera, as the heart. All fibers going direct to their destination are 
termed rami viscerates. 

The prevertebral ganglia, the semilunar, the renal, the superior and in- 
ferior mesenteric, give off fibers which pass to the walls of the blood- 
vessels and to the viscera of the stomach, intestine, gall-bladder, liver, 
kidney and pelvic viscera, etc. 

The peripheral ganglia, the ciliary, the spheno-palatine, the otic, the 
submaxillary, the cardiac, pelvic, etc., give off branches which pass to the 
non-striated muscle fibers in the organs to which they are in anatomic 
relation. 

From the distribution of the branches emerging from all the different 
groups of ganglia, there is reason to believe that they are directly associated 
with vaso-augmentor and vaso-inhibitor, viscero-augmentor and viscero- 
inhibitor, secret o-motor and secrete -inhibitor phenomena. 

The Anatomic Relation of the Central Nerve System to the Sympa- 
thetic Ganglia. — The central nerve system is associated anatomically and 
physiologically with the sympathetic ganglia through the intermediation 
of fine medullated nerve-fibers, the preganglionic, which have their 
origin in nerve-cells situated in four different regions, viz.: 

1. The Mid -brain Region. — The preganglionic nerve-fibers that leave 
the brain in this region arise from groups of nerve-cells situated high up in 
the gray matter beneath the aqueduct of Sylvius just where it widens to 
form the cavity of the third ventricle. From this origin they enter the 
trunk of the oculo-motor nerve and in association with it enter the orbit 
cavity. In this situation these preganglionic fibers leave the oculo- 
motor nerve and enter the ciliary or ophthalmic ganglion around the nerve- 
cells of which their terminal branches arborize. The gray postganglionic 
fibers arising in the gray cells of this ganglion enter the eyeball and are ulti- 
mately distributed to the sphincter muscle of the iris and to the ciliary 
muscle. 

2. The Bulbar Region. — The preganglionic fibers that leave the brain 
in this region arise from nerve-cells situated in the gray matter beneath the 
floor of the fourth ventricle a little above and below the calamus scrip- 
torius. These fibers leave this region by three routes, viz.: 

(a.) By way of the nerve of Wrisberg or the pars intermedia. The pre- 
ganglionic fibers that emerge in this nerve enter the facial nerve and 
subsequently pass by way of the great superficial petrosal nerve to the 
sphenopalatine ganglion, and by the way of the chorda tympani nerve to 



204 HUMAN PHYSIOLOGY 



the sub-maxillary ganglion, around the nerve-cells of which their terminal 
branches arborize. The gray postganglionic fibers which arise in the cells 
of these ganglia are distributed to the blood-vessels and glands of the 
nose and mouth and to the blood-vessels and epithelium of the sub- 
maxillary and sublingual glands respectively. 

(Jb.) By way of the glosso-pharyngeal nerve. The fibers that emerge in 
this nerve pass into the tympanic branch or nerve of Jacobson and ulti- 
mately arborize around the cells of the otic ganglion. The gray post- 
ganglionic fibers which arise in the cells of this ganglion pass by way of the 
auriculo-temporal branch of the trigeminal nerve to the blood-vessels and 
epithelium of the parotid gland. 

(c.) By way of the vagus nerve. The preganglionic fibers that leave in 
the trunk of the vagus nerve are ultimately distributed to the ganglia of 
the heart, stomach, small intestine, etc., around the nerve-cells of which 
their terminal branches arborize. The gray postganglionic fibers which 
arise in these ganglia pass to the heart-fibers, to the non-striated muscle- 
fibers in the walls of the stomach, intestines, etc. These fibers contained 
in the facial, glosso-pharyngeal and vagus nerves, together with their 
ganglionic continuations, have collectively been termed the bulbar auto- 
nomic system. Together with the fibers in the oculo-motor nerve they 
have been termed the cranio-bulbar autonomic system. 

3. The Mid-spinal Cord Region. — The preganglionic nerve-fibers that 
leave the spinal cord in this region arise from groups of nerve-cells situated 
in the gray matter between the levels of origin of the second thoracic 
and the second and third, perhaps the fourth, lumbar nerves. From this 
origin the fine pre-ganglionic fibers emerge from the cord in the ventral 
roots of the thoracic and upper lumbar nerves and hence naturally fall into 
two groups, viz.: the thoracic and the lumbar. Both groups of nerves 
accompany the ventral motor roots of the spinal nerves to about the point 
where each nerve divides into an anterior and a posterior branch; they then 
leave and enter the vertebral chain of ganglia. The branches of communi- 
cation are known as the white rami communicantes. The nerve-fibers 
composing these communicating branches terminate for the most part 
around the nerve-cells of the ganglia at the same and at somewhat different 
levels and in different regions. 

Some of the fibers of the thoracic group, however, cross the vertebral 
chain and then pass forward and downward, uniting to form the greater 
and lesser splanchnic nerves, the terminal branches of which arborize around 
the cells of the semilunar, the renal and the superior mesenteric ganglia. 
Some of the lumbar nerves also pass across the vertebral chain to form the 



. 



AUTONOMIC NERVE SYSTEM 205 

inferior splanchnic nerves , the terminal branches of which arborize around 
the cells of the inferior mesenteric ganglion. 

The distribution of the postganglionic fibers has already been alluded 
to. The preganglionic nerve-fibers having their origin in the mid-spinal 
cord region comprise all the vaso-motor (constrictor) nerves, the secreto- 
motor (sweat) nerves and the viscero-inhibitor nerves for the stomach, 
intestines and other viscera, as well as some viscero-motor fibers. 
These nerves collectively constitute the thoracico-lumbar autonomic nerve 
system. 

4. The Sacral Spinal-cord Region. — The preganglionic nerve-fibers 
that leave the spinal cord in this region arise from groups of nerve-cells situ- 
ated in the gray matter between and including the levels of origin of the 
second, the third and the fourth (?) sacral nerves. From this origin the 
preganglionic fibers emerge from the cord in association with the large 
motor fibers composing the ventral roots of these sacral nerves and pass 
with them to the interior of the pelvis. Here they leave the sacral 
nerves and enter the pudendal or pelvic nerve (the nervus erigens) and 
finally terminate around the cells of the pelvic ganglia. From these gan- 
glia postganglionic fibers arise which pass onward to be distributed to the 
non-striated muscle-fibers of pelvic viscera and the blood-vessels of the 
external generative organs. These fibers contained in the sacral nerves 
together with their post-ganglionic continuation have collectively been 
termed the sacral autonomic system. It may be regarded as a special 
nerve system for the anal end of the gut and structures developmentally 
connected with it. 

The Functions of the Autonomic Nerve System. — The functions of 
the autonomic nerve system, as determined from its anatomic distribution, 
and the results of experimental investigations, are to augment or to inhibit 
the tonus of the blood-vessels including the heart, the tonus of visceral 
walls and the activity of the epithelium of glands, and are, therefore, the 
sum total of the functions of the vaso-motor, viscero-motor and secreto- 
motor nerves, that is, the nerves which collectively constitute this system. 

In the various sections of the text specific statements are to be found as 
to the functions of the autonomic nerves in association with the oculo- 
motor nerve, the nerve of Wrisberg, (the great petrosal and chorda tympani 
fibers) the glosso-pharyngeal nerve (Jacobson's nerve) the vagus nerve 
(the cardiac, bronchial, gastric and intestinal fibers) the thoracico-lumbar 
nerves (the vaso-motor, viscero-motor, secreto-motor, and cardiac 
accelerator fibers) the sacral nerves (the vaso-dilatator and viscero- 
motor, and inhibitor fibers for the pelvic viscera and external organs of 
generation. 



206 HUMAN PHYSIOLOGY 

THE CRANIAL NERVES 

The cranial nerves come off from the base of the brain, pass through fora- 
mina in the walls of the cranium, and are distributed to the structures of 
the head, the face and in part to the organs of the thorax and abdomen. 

According to the classification of Soemmering, there are twelve pairs of 
nerves, enumerating them from before backward, as follows — viz : 

First nerve, or olfactory. Seventh nerve, or facial, portio dura. 

Second nerve, or optic. Eighth nerve, or acoustic. 

Third nerve, or motor oculi com- Ninth nerve, or glossopharyngeal. 

munis. Tenth nerve, or pneumogastric. 

Fourth nerve, or trochlearis. Eleventh nerve, or spinal accessory. 

Fifth nerve, or trigeminal. Twelfth nerve, or hypoglossal. 
Sixth nerve, or abducent. 

The cranial nerves may also be classified physiologically, according to 
their function, into three groups : 

i. Nerves of special sense — e.g., olfactory, optic, acoustic, gustatory, 
glosso-pharyngeal and chorda tympani). 

2. Nerves of motion — e.g., motor oculi, pathetic, small root of the 
trigeminal, abducent, facial, spinal accessory and hypoglossal. 

3. Nerves of general sensibility — e.g., large root of the trigeminal, the 
glosso-pharyngeal and the pneumogastric. 

ORIGINS OF THE CRANIAL NERVES 

The nerves of special sense have their origin in neuro-epithelial cells in the 
sense organs with which they are connected. 

The nerves of motion have their origin in nerve-cells situated in the gray 
matter beneath the floor of the aqueduct of Sylvius and the floor of the 
fourth ventricle. 

The nerves of general sensibility have their origin in the ganglia situated 
on their trunks. 

First Nerve — Olfactory 

The olfactory nerve is situated in the upper third of the nasal fossa. It 
consists of from 20 to 30 branches. 

Origin. — From neuro-epithelial cells situated among the epithelial cells 
covering the mucous membrane. From these cells the nerve-fibers pass 
upward through foramina in the cribriform plate of the ethmoid bone and 
arborize around nerve-cells, in the olfactory bulb. 






THE CRANIAL NERVES 207 

The Olfactory Tract. — The olfactory tract consists of both gray and 
white fibers which pass from their origin in the bulb, to the base of the cere- 
brum where it divides into three branches, viz. : an external white root, which 
passes across the fissure of Sylvius to the middle lobe of the cerebrum; an 
internal white root, which passes also into the middle lobe; a gray root, which 
is in relation with the anterior lobe. The white fibers at least terminate 
around nerve-cells in the gray matter of the pre-callosal part of the gyrus 
fornicatus, the gyrus hippocampus and the gyrus uncinatus. 

Properties. — The olfactory nerves do not give rise to either motor or 
sensor phenomena when stimulated. When stimulated at their periphery 
by odorous particles, nerve impulses are developed which, when conducted 
to the brain, evoke the sensation of smell. Destruction of the olfactory 
nerves, the bulb or tract, is followed by a loss of the sense of smell. 

Function. — Presides over the sense of smell. Conducts impulses to the 
cerebrum which give rise to sensations of odor. 



Second Nerve — Optic 

Origin. — The optic nerve arises from large nerve-cells in the anterior 
part of the retina. From this origin the nerve-fibers turn backward and 
converge to form a well-defined bundle (the optic nerve) which passes out 
of the eyeball, through the orbit cavity as far as the sella turcica. At this 
point there is a union and partial decussation, in man at least, of the 
fibers, forming what is known as the optic chiasm. From the posterior 
portion of the chiasm there passes backward on either side a bundle of 
nerve-fibers, the optic tract. Each tract contains nerve-fibers, which 
come from the temporal two-thirds of the retina of the same side and 
the nasal third of the retina of the opposite side. The fibers of the 
optic tract arborize around nerve-cells in the external geniculate body, 
the pulvinar, and the anterior quadrigeminal body. By means of the 
optic radiation, the nerve-cells in these different ganglia are brought into 
relation with the visual center, the cuneus. 

Properties. — The optic nerves are insensible to ordinary impressions, 
and convey only the special impressions of light. Division of one of the 
nerves is attended by complete blindness in the eye of the corresponding 
side. 

Hemiopia and Hemianopsia. — Owing to the decussation of the fibers in 
the optic chasm, division of the optic tract produces loss of sight in the 
outer half of the eye of the same side, and in the inner half of the eye of the 



208 HUMAN PHYSIOLOGY 






opposite side, the blind part being separated from the normal part by a 
vertical line. The term hemiopia is applied to the loss of function or 
paralysis of the one-half of the retina; as a result of this, there will be an 
obliteration of the field of vision on the opposite side to which the term 
hemianopsia is given. If, for example, the right optic tract be divided, 
there will be hemiopia in the outer half of the right eye and inner half of 
the left eye, thus causing left lateral hemianopsia,- and as the two halves 
are affected which correspond in normal vision, the condition is known 
as homonymous hemianopsia. Lesion of the anterior part of the optic 
chiasm caused blindness in the inner half of the two eyes. 

Functions. — Governs the sense of sight. Receives and conveys to 
the brain the nerve impulses made by ether vibrations and which give 
rise to the sensation of light. 

The reflex movements of the iris are called forth by stimulation of the 
optic nerve. When light falls upon the retina, the nerve impulse devel- 
oped is carried back to the tubercula quadrigemina, where it is trans- 
formed into a motor impulse, which then passes outward through the motor 
oculi nerve to the contractile fibers of the iris and diminishes the size of 
the pupil. The absence of light is followed by a dilatation of the pupil. 

Third Nerve— The Oculo-Motor 

Origin. — From several groups of nerve-cells situated in the gray matter 
beneath the aqueduct of Sylvius. 

Distribution. — From this origin the nerve- fibers pass forward and emerge 
from the cerebrum at the inner side of the crus cerebri. The nerve then 
passes forward, and enters the orbit through the sphenoid fissure, where it 
divides into a superior branch distributed to the superior rectus and lev- 
ator palpebrce muscles; and inferior branch, sending branches to the internal 
and inferior recti and the inferior oblique muscles; filaments also pass into 
the cilary or ophthalmic ganglion; from this ganglion the ciliary nerves arise, 
which enter the eyeball and are distributed to the circular fibers of the iris 
and the ciliary muscle. The third nerve also receives filaments from the 
cavernous plexus of the sympathetic and from the fifth nerve. 

Properties. — Irritation of the root of the nerve produces contraction of 
the pupil, internal strabismus, and muscular movements of the eye, but no 
pain. Division of the nerve is followed by ptosis (falling of the upper 
eyelid) ; external strabismus, due to the unopposed action of the external 
rectus muscle; paralysis of the accommodation of the eye; dilatation of the 



THE CRANIAL NERVES 209 

pupil from paralysis of the circular fibers of the iris and ciliary muscle; 
and inability to rotate the eye, slight protrusion, and double vision. The 
images are crossed; that of the paralyzed eye is a little above that of the 
second, and its upper end inclined toward it. 

Function. — Governs movements of the eyeball by innervating all the 
muscles except the external rectus and superior oblique, influences the 
movements of the iris, elevates the upper lid, influences the accommoda- 
tion of the eye for distances. Can be called into action by (1) voluntary 
stimuli, (2) by reflex action through irritation of the optic nerve. 

Fourth Nerve — Trochlearis 

Origin. — From nerve-cells situated in the gray matter beneath the 
aqueduct of Sylvius, just posterior to the last nucleus of the third nerve. 

Distribution. — The nerve enters the orbital cavity through the sphenoid 
fissure, and is distributed to the superior oblique muscle; in its course it 
receives filaments from the ophthalmic branch of the fifth pair and the 
sympathetic. 

Properties. — When the nerve is irritated, muscular movements are pro- 
duced in the superior oblique muscle, and the pupil of the eye is turned 
downward and outward. Division or paralysis lessens the movements 
and rotation of the globe downward and outward. The diplopia conse- 
quent upon this paralysis is homonymous, one image appearing above 
the other. The image of the paralyzed eye is below, its upper end in- 
clined toward that of the sound eye. 

V 

Function. — Governs the movements of the eyeball produced by the 

action of the superior oblique muscles. 

Sixth Nerve* — Abducent 

Origin. — From nerve-cells situated beneath the upper half of the floor 
of the fourth ventricle. 

Distribution. — From this origin the nerve passes into the orbit through 
the sphenoid fissure, and is distributed to the external rectus muscle. 
Receives filaments from the cervical portion of the sympathetic, through 
the carotid plexus, and spheno-palatine ganglion. 

•The sixth nerve is considered in connection with the third and fourth nerves 
since they together constitute the motor apparatus by which the. ocular muscles are 
excited to action. 
14 



2IO HUMAN PHYSIOLOGY 

Properties. — When irritated, the external rectus muscle is thrown into 
convulsive movements and the eyeball is turned outward. When divided 
or paralyzed, this muscle is paralyzed, motion of the eyeball outward 
past the median line is impossible, and the homonymous diplopia increases 
as the object is moved outward past this line. The images are upon the 
same plane and parallel. Internal strabismus results because of the un- 
opposed action of the internal rectus. 

Function. — To innervate the external rectus muscle by which the eye- 
ball is turned outward. 

Fifth Nerve — Trigeminal 

The fifth nerve consists of both afferent and efferent fibers which for 
the most part are separate and distinct. The afferent fibers constitute by 
far the major portion, the efferent fibers the minor portion of the nerve. 

Origin of the Afferent Fibers. — The afferent fibers have their origin in 
nerve-cells in the Gasserian ganglion. From each cell a short process 
develops which soon divides into two branches, one of which passes cen- 
trally, the other peripherally. The centrally directed branches form the 
so-called large root; the peripherally directed branches collectively 
constitute the three main divisions of the nerve, viz.: the ophthalmic, 
the superior maxillary and the inferior maxillary. 

Distribution. — The centrally directed branches enter the pons Varolii 
on its lateral aspect. After pursuing a short distance, these fibers arborize 
around nerve-cells in the gray matter of the pons and medulla. 
The peripherally directed branches are distributed as follows: 
i. The ophthalmic branches to the conjunctiva and skin of the upper 
eyelid, the cornea, the skin of the forehead and the nose, the lachrymal 
gland and the mucous membrane of the nose. 

2. The superior maxillary branches to the skin and conjunctiva of the 
lower lid, the nose, the cheek and upper lip, the palpate teeth of the upper 
jaw and the alveolar processes. 

3. The inferior maxillary branches to the external auditory meatus, the 
side of the head, the mouth, the tongue, the teeth of the lower jaw, the 
alveolar processes and the skin of the lower part of the face. 

Properties. — The trigeminal nerve, composed mainly of afferent fibers, 
is the most acutely sensitive nerve in the body, and endows all the parts to 
which it is distributed with general sensibility. 

Stimulation of the large root, or of any of its branches, will give rise to 



THE CRANIAL NERVES 211 

marked evidence of pain; the various forms of neuralgia of the head and 
face being occasioned by compression, disease, or exposure of some of 
its terminal branches. 

Division of the large root within the cranium is followed at once by a 
complete abolition of all sensibility in the head and face, but is not at- 
tended by any loss of motion. The integument, the mucous membranes, 
and the eye may be lacerated, cut, or bruised, without the animal exhib- 
iting any evidence of pain. At the same time the lachrymal secretion is 
diminished, the pupil becomes contracted, the eyeball is protruded, and 
the sensibility of the tongue is abolished. 

The reflex movements of deglutition are also somewhat impaired, the 
impressions of the food being unable to reach and excite the nerve center 
in the medulla oblongata. 

Origin of the Efferent Fibers. — The efferent fibers have their origin in 
nerve-cells in the gray matter of the pons Varolii beneath the floor of the 
fourth ventricle. 

Distribution. — The efferent fibers, known collectively as the small root, 
emerge from the side of the pons Varolii, pass forward beneath the ganglion 
of Gasser, beyond which they enter the inferior maxillary division. After 
a short course most of these fibers leave the common trunk and are dis- 
tributed to the muscles of mastication, viz. : the temporal, the masseter, 
the internal and external pterygoid muscles. Other fibers are distributed 
to the mylohyoid muscle, the tensor palati and the tensor tympani muscles. 

Properties. — Stimulation of the small root produces convulsive move- 
ments of the muscles of mastication; section of the root causes paralysis 
of these muscles, after which the jaw is drawn to the opposite side by the 
action of the opposing muscles. 

The Influence of the Trigeminal on the Special Senses. — After division 
of the large root within the cranium, a disturbance in the nutrition of the 
special senses sooner or later manifests itself. 

Sight. — In the course of twenty-four hours the eye becomes very vascular 
and inflamed, the cornea becomes opaque and ulcerates, the humors are 
discharged, and the eye is totally destroyed. 

Smell. — The nasal mucous membrane swells up, becomes fungous, and 
is liable to bleed on the slightest irritation. The mucus is increased 
in amount, so as to obstruct the nasal passages; the sense of smell is 
finally abolished. 

Hearing. — At times the hearing is impaired from disorders of nutrition 
in the middle ear and external auditory meatus. 



212 HUMAN PHYSIOLOGY 

Alteration in the nutrition of the special senses is not marked if the 
section is made posterior to the ganglion of Gasser and to the anastomos- 
ing filaments of the sympathetic, which joins the nerves at this point; 
but if the ganglion be divided, these effects are very noticeable, owing to 
the section of the sympathetic filaments. 

Function. — The trigeminal nerve, through its afferent fibers, endows all 
the parts of the head and face to which it is distributed with sensibility; 
through its efferent fibers it gives motion to the muscles of mastication, 
and to the tensor muscle of the palate and the tensor of the tympanic 
membrane; through anastomosing fibers from the sympathetic it influ- 
ences the nutrition of the special senses. 

Seventh Nerve — Facial Nerve 

Origin. — From a large nucleus of nerve-cells situated in the gray matter 
beneath the upper half of the floor of the fourth ventricle. 

Distribution. — From this origin the nerve emerges from the lower 
border of the pons. It then passes into the internal auditory meatus in 
company with the nerve of Wrisberg, and then enters the aqueduct of 
Fallopius. 

The nerve-fibers composing the nerve of Wrisberg have their origin in 
nerve-cells in the geniculate ganglion, situated on the facial just where it 
bends to enter the aqueduct of Fallopius. The centrally directed branches 
enter the medulla oblongata around the nerve-cells of which they termin- 
ate; the peripherally directed branches enter the trunk of the facial. 

In the aqueduct the facial gives off the following branches — viz. : 

i. The large petrosal nerve, which passes forward to the spleno palatine, 
or Meckel's ganglion. 

2. The small petrosal nerve, which passes to the otic ganglion. 

3. The tympanic branch, which passes to the stapedius muscle and en- 
dows it with motion. 

4. The chorda tympani nerve, which, after entering the posterior part of 
the tympanic cavity, passes forward between the malleus and incus, 
through the Glasserian fissure, and joins the .lingual branch of the fifth 
nerve. It is then distributed to the mucous membrane of the anterior two- 
thirds of the tongue and the submaxillary glands. 

After emerging from the stylomastoid foramen, the facial nerve sends 
branches to the muscles of the ear, the occipitofrontalis, the digastric, the 
palatoglossi, and palatopharyngeal; after which it passes through the paro- 



THE CRANIAL NERVES 213 

tid gland and divides into the temporofacial and cervicofacial branches, 
which are distributed to the superficial muscles of the face — viz., occipito- 
frontal, corrugator supercilii, orbicularis palpebrarum, levator labii 
superioris et alaeque nasi, buccinator, levator anguli oris, orbicularis oris, 
zygomatici, depressor anguli oris, platysma myoides, etc. 

Properties. — The facial is a motor nerve at its origin, but in its course 
receives sensitive filaments from the fifth pair and the pneumogastric. 

Stimulation of the nerve, after its emergence from the stylomastoid fora- 
men, produces convulsive movements in all the superficial muscles of the 
face. Division of the nerve at this point causes paralysis of these muscles 
on the side of the section, constituting facial paralysis, the phenomena of 
which are a relaxed and immobile condition of the same side of the face, 
the eyelids remain open, from paralysis of the orbicularis palpebrarum; the 
act of winking is abolished; the angle of the mouth droops, and saliva con- 
stantly drains away: the face is drawn over to the second side; the face be- 
comes distorted upon talking or laughing; mastication is interfered with, 
the food accumulating between the gums and cheek, from paralysis of the 
buccinator muscle; fluids escape from the mouth in drinking; articulation 
is impaired, the labial sounds being imperfectly pronounced. 

Properties and Functions of the Branches Given off in the Aqueduct 
of Fallopius. 

1. The large petrosal, when stimulated, gives rise to a dilatation of the 
blood-vessels and a secretion from the mucous membrane of nose, soft 
palate, upper part of the pharynx, roof of the mouth, and gums. It there- 
fore contains vaso-motor and secretor fibers, which are in relation with the 
spheno-palatine ganglion. 

2. The tympanic branch causes the stapedius muscle to contract. 

3. The chorda tympani influences the circulation of the blood around, 
and the secretion of saliva from, the submaxillary glands, and through the 
nerve of Wri.^bcr^ endows the anterior two-thirds of the tongue with the 
sense of taste. Stimulation of the chorda tympani dilates the blood- 
vessels, increases the quantity and rapidity of the stream of blood, and 
increases the secretion of saliva. Division of the nerve is followed by 

I contraction of the ve-seK and arrest of the secretion, and a loss of the sense 
1 of taste on tl idc. It therefore contains vaso-motor, secretor and 

, gustatory nerve -fil 

Function. — The facial is the nerve of expression, and coordinates the 
' muscles employed to delineate the various emotions, influences the sense of 
~te and the secretions of the submaxillary and sublingual glands. 



214 HUMAN PHYSIOLOGY 

Eighth Nerve — Acoustic Nerve 

The eighth nerve consists of two portions, a cochlear or auditory and a 
vestibular or equilibratory. 

Origin. — The cochlear portion has its origin in the bipolar nerve-cells of 
the spinal ganglion located in the spiral canal near the base of the osseous 
lamina spiralis. The vestibular portion has its origin in the bipolar nerve- 
cells of the ganglion of Scarpa located in the internal auditory meatus. 

Distribution. — The common trunk of the eighth nerve, consisting of 
both the cochlear and vestibular portions, emerges from the internal audi- 
tory meatus, after which it passes backward and inward as far as the lateral 
aspect of the pons, where the two main divisions again separate. The 
cochlear portion passes to the outer side of the restiform body; the vestibu- 
lar portion passes to the inner side of the restiform body to the dorsal por- 
tion of the pons. After entering the pons the fibers composing both por- 
tions come into histologic relations with different groups of nerve-cells. 

Properties. — Stimulation of the cochlear nerve is<unattended by either 
motor or sensor phenomena. Division of the nerve is followed by a loss of 
hearing. Destruction of the semicircular canal, involving a lesion of the 
vestibular nerves at their origin, is followed by an impairment of the power 
of coordination and equilibration. 

Functions. — The cochlear nerve presides over the sense of hearing. It 
carries to the brain the nerve impulses produced by the impact of" atmos- 
pheric vibrations on the ear, and which give rise to the sensation of sound. 
The vestibular nerve carries nerve impulses to the brain, which excite cer- 
tain reflex adaptive movements by which the equilibrium of the body is 
maintained. 

Ninth Nerve — Glossopharyngeal 

Origin. — From nerve-cells in the ganglia situated on the trunk of the 
nerve near the medulla oblongata — viz., the petrosal ganglion and the 
jugular ganglion. From these cells a single branch emerges, which soon 
divides into two branches, one of which passes centrally, the other pe- 
ripherally. The centrally directed branches enter the medulla oblongata, 
where they s terminate around nerve-cells. The peripherally directed 
branches collectively form the two main divisions from which the nerve 
takes its name. 

The glossopharyngeal also contains efferent nerve-fibers, which have 
their origin in nerve-cells beneath the floor of the fourth ventricle. 



THE CRANIAL NERVES 21 5 

Distribution. — The trunk of the nerve passes downward and forward, 
receiving near the jugular ganglion fibers from the facial and pneumogas- 
tric nerves. It divides into two large branches, one of which is distributed 
to the base of the tongue, the other to the pharynx. In its course it sends 
filaments to the otic ganglion; a tympanic branch which gives sensibility to 
the mucous membrane of the fenestra rotunda, fenestra ovalis, and 
Eustachian tube; lingual branches to the base of the tongue; palatal 
branches to the soft palate, uvula, and tonsils; pharyngeal branches to 
the mucous membrane of the pharynx. 

Properties. — Irritation of the roots at their origin calls forth evidences 
of pain; it is, therefore, a sensor nerve, but its sensibility is not so acute as 
that of the trigeminal. Irritation of the trunk after its exit from the 
cranium produces contraction of the muscles of the palate and pharynx, 
owing to the presence of motor fibers. 

Division of the nerve abolishes sensibility in the structures to which it is 
distributed and impairs the sense of taste in the posterior third of the 
tongue (see Sense of Taste). 

Function. — Governs the sensibility of the pharynx, presides partly over 
the sense of taste, and controls reflex movements of deglutition and 
vomiting. 

Tenth Nerve — Pneumogastric. Vagus 

Origin. — From the nerve-cells situated along the trunk of the nerve near 
the medulla oblongata — viz. : the jugular and the plexiform ganglia. From 
the nerve-cells in these ganglia a short process emerges which soon divides 
into two branches one of which passes centrally, the other peripherally. 
The central branches enter the medulla oblongata, where they terminate 
around nerve-cells; the peripheral branches collectively form the main 
portion of the trunk of the nerve. 

The pneumogastric also contains efferent fibers which have their origin 
in nerve-cells beneath the floor of the medulla oblongata. It also receives 
motor fibers from the spinal accessory, the facial, the hypoglossal and the 
anterior branches of the two upper cervical nerves. 

Distribution. — As* the nerve passes down the neck it sends off the follow- 
ing main branches: 

1. Pharyngeal hich assist in forming the pharyngeal plexus 
which is distributed to the mucous membrane and to the muscles of the 
pharynx. 

2. Superior laryngeal nerve, which enters the larynx through the 



2l6 HUMAN PHYSIOLOGY 

thyrohyoid membrane, and is distributed to the mucous membrane lining 
the interior of the larynx, and to the cricothyroid muscle and the inferior 
constrictor of the pharynx. The "depressor nerve" found in the rabbit, 
is formed by the union of two branches, one from the superior laryngeal, 
the other from the main trunk; it passes downward to be distributed to 
the heart. 

3. Inferior laryngeal, which sends its ultimate branches to all the in- 
trinsic muscles of the larynx except the cricothyroid, and to the inferior 
constrictor of the pharynx. 

4. Cardiac branches given off from the nerve throughout its course 
which unite with the sympathetic fibers to form the cardiac plexus, to 
be distributed to the heart. 

5. Pulmonary branches, which form a plexus of nerves, and are dis- 
tributed to the bronchi and their ultimate terminations, the lobules and 
air cells. 

From the right pneumogastric nerve branches are distributed to the 
mucuous membrane and the muscular coats of the stomach and intestines, 
and to the liver, spleen, kidneys, and suprarenal capsules. 

Properties. — At its origin the pneumogastric nerve is sensory, as shown 
by direct irritation or galvanization, though its sensibility is not very 
marked. In its course it exhibits motor properties, from anastomosis with 
motor nerves. 

The pharyngeal branches assist in giving sensibility to the mucous 
membrane of the pharynx, and influence reflex phenomena of degluti- 
tion through motor fibers which they contain, derived from the spinal 
accessory. 

The superior laryngeal nerve endows the upper portion of the larynx 
with sensibility; protects it from the entrance of foreign bodies; by con- 
ducting impressions to the medulla, excites the reflex movements of 
deglutition and respiration; through the motor filaments it contains, 
produces contraction of the cricothyroid muscle. 

Division of the "depressor nerve" and stimulation of the central end 
retard the pulsations of the heart, and by depressing the vaso-motor 
center, diminish the pressure of blood in the large vessels, by causing 
dilatation of the intestinal vessels through the splanchnic nerves. 

The inferior laryngeal contains, for the most part, motor fibers from the 
spinal accessory. When irritated, produces movement in the laryngeal 
muscles. When divided, is followed by paralysis of these muscles, except 
the cricothyroid, impairment of phonation, and an embarrassment of the 
respiratory movement of the larynx, and, finally, death from suffocation. 



THE CRANIAL NERVES 21 7 

The cardiac branches, through filaments derived from the spinal acces- 
sory, or possibly from the medulla oblongata direct, exert a direct inhibi- 
tory action upon the heart. Division of the pneumogastrics or vagi in 
the neck is followed by increased frequency of the heart's action. Stimu- 
lation of the peripheral ends diminishes the heart's pulsations, and, if 
sufficiently powerful, arrests it in diastole. 

The pulmonary branches give sensibility to the bronchial mucous 
membrane and govern the movements of respiration. Division of both 
pneumogastrics in the neck diminishes the frequency of the respiratory 
movements, which may fall as low as four to six a minute; death usually 
occurs in from five to eight days. Feeble stimulation of the central 
ends of the divided nerves accelerates respiration, powerful stimulation 
retards, and may even arrest the respiratory movements. 

The gastric branches give sensibility to the mucous coat, and through 
motor or efferent fibers give motion to the muscular coat of the stomach. 
They influence the secretion of gastric juice, and aid the process of 
digestion. 

The intestinal branches give sensibility and motion to the small 
intestines. 

Function. — A great sensor nerve, which, through filaments from motor 
sources, influences deglutition, the action of the heart, the circulatory 
and respiratory systems, voice, the secretions of the stomach, intestines, 
and various glandular organs, and the contraction of the walls of the 
stomach and intestines. 

Eleventh Nerve — Spinal Accessory 

The spinal accessory nerve consists of two distinct portions, the med- 
ullary or bulbar, and the spinal. 

Origin. — The medullary portion has its origin in nerve-cells in the 
lower part of the nucleus ambiguus, located beneath the floor of the fourth 
ventricle. From this origin the nerve-fibers pass forward and emerge 
from the medulla oblongata on its lateral aspect. 

The spinal portion has its origin in the nerve-cells located in the lateral 
gray matter of the spinal cord as far down as the fifth cervical nerve. 
From this origin the nerve-fibers pass to the surface of the cord to emerge 
between the ventral and dorsal roots in from six to eight filaments, after 
which they unite to form a well-defined nerve. It then passes into the 
cranial cavity through the foramen magnum and unites with the medul- 
lary portion. 



2l8 HUMAN PHYSIOLOGY 

Distribution. — After the union the common trunk emerges from the 
cranial cavity through the jugular foramen and after sending branches 
to the pneumogastric and receiving other in turn from the pneumogastric 
as well as from the upper cervical nerves it divides into two branches — 
viz. : 

i. An internal or anastomotic branch which soon enters the trunk of the 
pneumogastric nerve. The fibers of this branch are ultimately distrib- 
uted to some of the pharyngeal muscles; to all of the muscles of the larynx 
by way of the laryngeal branches of the vagus nerve, and, according to 
most authorities, to the heart. 

2. An external branch consisting chiefly of the accessory fibers from the 
spinal cord. It is distributed to the sterno-cleido-mastoid and trapezius 
muscles. 

Properties. — At its origin it is a purely motor nerve, but in its course 
it exhibits some sensibility, due to the presence of anastomosing fibers. 

Destruction of the medullary root — e.g., tearing it from its attachment by 
means of forceps, impairs the action of the muscles of deglutition and 
destroys the power of producing vocal sounds from paralysis of the laryn- 
geal muscles, without, however, interfering with the respiratory move- 
ments of the larynx, these being controlled by other motor nerves. The 
normal rate of movement of the heart is increased by destruction of the 
medullary root. 

Irritation of the external branch throws the trapezius and sternomastoid 
muscles into convulsive movements, though section of the nerve does not 
produce complete paralysis, as they are also supplied with motor influence 
from the cervical nerves. The sternomastoid and trapezius muscles per- 
form movements antagonistic to those of respiration, fixing the head, 
neck, and upper part of the thorax, and delaying the expiratory movement 
during the acts of pushing, pulling, straining, etc., and in the production 
of a prolonged vocal sound, as in singing. When the external branch 
alone is divided, in animals, they experience shortness in breath during 
exercise, from a want of coordination of the muscles of the limbs and 
respiration; and while they can make a vocal sound, it cannot be 
prolonged. 

Function. — Governs phonation by its influence upon the muscles 
regulating the position and tension of the vocal bands; influences the 
movements of deglutition, inhibits the action of the heart, and controls 
certain respiratory movements associated with sustained or prolonged 
muscular efforts and phonation. 



SENSE OF TOUCH 219 

Twelfth Nerve — Hypoglossal 

Origin. — From nerve-cells situated deep in the substance of the medulla 
oblongata, on a level with the lowest portion of the floor of the fourth 
ventricle. From this origin the fibers pass forward and emerge from the 
medulla in the groove between the anterior pyramid and the olivary body. 

Distribution. — The trunk formed by the union of the different filaments 
passes out of the cranial cavity through the anterior condyloid foramen. 
After emerging from the cranium, it sends filaments to the sympathetic 
and pneumogastric; it anastomoses with the lingual branch of the fifth 
pair, and receives and sends filaments to the upper cervical nerves. The 
nerve is finally distributed to the sternohyoid, sternothyroid, omohyoid, 
thyrohyoid, styloglossi, hyoglossi, geniohyoid, geniohyoglossi, and the 
intrinsic muscles of the tongue. 

Properties. — A purely motor nerve at its origin, but derives sensibility 
outside the cranial cavity from anastomosis with the cervical pneumo- 
gastric, and fifth nerves. 

Irritation of the nerve gives rise to convulsive movements of the tongue 
and slight evidences of sensibility. 

Division of the nerve on both sides abolishes all movements of the tongue 
and interferes considerably with the act of deglutition. 

When the hypoglossal nerve is involved in hemiplegia, the tip of the 
tongue is directed to the paralyzed side when the tongue is protruded, 
owing to the unopposed action of the geniohyoglossus on the sound side. 

Articulation is considerably impaired in paralysis of this nerve, great 
difficulty being experienced in the pronunciation of the consonantal 
sounds. 

Mastication is performed with difficulty, from inability to retain the 
food between the teeth until it is completely triturated. 

Function. — Governs all the movements of the tongue and influences the 
functions of mastication, deglutition and articulation. 

THE SENSE OF TOUCH 

Touch may be defined as the sense by which pressure or traction on the 
skin and mucous membrane is perceived. 

The physiologic mechanism involved in the sense of touch includes the 
skin and the mucous membrane lining the mouth, the afferent ncr 



2 20 HUMAN PHYSIOLOGY 

their cortical connections, and nerve-cells in the cortex of the parietal 
lobe. ** 

Peripheral excitation of this mechanism develops nerve impulses which, 
transmitted to the cortex, evoke sensations of touch and temperature. 
To the skin, therefore, is ascribed a touch sense and a temperature sense. 
Of the touch sensations two kinds may be distinguished: viz., pressure 
sensations and place sensations. With the contact of an external body 
there arises the perception not only of the pressure, but also the percep- 
tion of the place or locality of the contact. In accordance with this, it is 
customary to attribute to the skin a pressure sense and a location sense. 

The specific physiologic stimuli to the terminal organs in the skin and 
oral mucous membrane are mechanic pressure and thermic vibrations. 

The structure of the skin and the modes of termination of the sensory 
nerves have already been considered (see page 149). 

The touch sense is coextensive with the skin and the mucous membrane 
of the mouth. The touch areas, however, are not continuous but discrete 
and vary in number in each square centimeter of skin. Thus in the skin 
of the calf 15 touch spots or areas have been counted; in the palm of the 
hand 40 to 50. Stimulation with a fine bristle of such an area calls forth 
the sensation of touch. In the tip of the index finger the touch sense is 
quite acute and associated with the presence of touch corpuscles of which 
there are about 20 to each square millimeter of surface. 

The pressure sense varies with the sensitivity of the skin, which varies 
in different parts of the body in accordance with the size of the area 
pressed. 

The place or location sense is the localization of a sensation to the place 
stimulated. This holds true not only for two or more points near or 
widely separated on the same side, but also for corresponding points on 
opposite sides of the body, even when the stimuli have the same intensity 
and are simultaneously applied. 

The delicacy of the localizing power in any part of the skin is determined 
by testing the power which the part possesses of distinguishing the sensa- 
tions produced by the contact of the points of a pair of compasses placed 
close together. The distance to which the points must be separated in 
order to evoke two separate recognizable sensations is a measure of the 
diameter of the sensor circle. Within this circle the two sensations become 
fused into one sensation. The discriminative sensibility of different 
regions as determined by compass points is shown in the following table; 
the numbers represent the distances at which two sensations are 
recognized: 



SENSE OF TASTE 221 



Tip of tongue i . I 

Palmar surface of third phalanx of index-finger 2.2 

Red surface of lips 4.5 

Palmar surface of first phalanx of finger 5.5 

Tip of nose 6.8 

Palm of hand 8.9 

Lower part of forehead 22.6 

Dorsum of hand 31.6 

Dorsum of foot 40 . 6 

Middle of the back 67 . 7 

The temperature sense is the recognition of changes in the temperature 
of the skin produced in a variety of ways through the sensations of heat 
and cold. This sense depends on the fact that all over the skin there are 
small areas some of which respond to warm,, others to cold objects and are 
therefore called hot and cold spots. When stimulated they call forth 
sensations of heat and cold. 



THE SENSE OF TASTE 

The sense of taste may be denned as the sense by which the specific 
quality or flavor of a substance, applied to the taste organ, is perceived. 
This sense is located mainly in the mucous membrane covering the surface 
of the tongue. 

The physiologic mechanism involved in the sense of taste includes the 
tongue, the gustatory nerves (contained in the trunks of the chorda 
tympani and glosso-pharyngeal nerves) their cortical connections and 
nerve-cells in the gray matter of the sub-collateral convolution. The 
peripheral excitation of this apparatus gives rise to nerve impulses which 
transmitted to the brain evoke the sensations of taste. The specific 
physiologic stimulus is matter, organic and inorganic, in a state of solution. 

Taste Buds or Beakers. — The peripheral ends of the taste nerves are 
provided with small ovoid bodies termed taste buds or beakers. The wall 
of the bud is composed of elongated curved epithelium at one point of 
which there is a small opening or pore. The interior contains narrow 
spindle-shaped neuro-epithelial cells provided at their outer extremity 
with stiff-hair like filaments which project into the taste pore. These 
neuro-epithelial cells are in histologic connection with the nerves of taste. 

The Taste Area. — The taste area, though confined for the most part 
to the tongue, extends in different individuals to the mucous membrane 
of the hard palate, to the anterior surface of the soft palate, to the uvula, 



22 2 HUMAN PHYSIOLOGY 

the anterior and posterior half arches, the tonsils, the posterior wall of 
the pharynx, and the epiglottis. 

The Taste Sensations. — The sensations which arise in consequence of 
impressions made by different substances on the peripheral apparatus of 
this area are in so many instances combinations of taste, touch, tempera- 
ture and smell that they are extremely difficult of classification. Never- 
theless six primary tastes can be recognized: bitter, sweet, acid or sour, 
salt or saline, alkaline and metallic. Though the contact of any bitter, 
sweet, acid, salt, etc., substance with any part of the tongue will, if the 
substance be present in sufficient quantity or concentration, develop a 
corresponding sensation, some regions of the tongue are more sensitive 
and responsive than others. Thus, the posterior portion is more sensitive 
to bitter substances than the anterior; the reverse is true for sweet sub- 
stances and perhaps for acids and salines. 

The intensity of the resulting sensation in any given instance will depend 
on the degree of concentration of the substance, while its massiveness will 
depend on the area affected. 

The essential conditions for the production of the sensations of taste 
are: 

i. A state of solubility of the food. 

2. A free secretion of the saliva, and 

3. Active movements on the part of the tongue, exciting pressure 
against the roof of the mouth, gums, etc., thus aiding the solution of 
various articles and their entrance into the taste beakers. 

THE SENSE OF SMELL 

The sense of smell is the sense by which certain qualities of substances 
entering the nose are perceived. 

The physiologic mechanism involved in the sense of smell includes the 
nasal fossae, the olfactory nerves, the olfactory tracts, and nerve-cells in 
those areas of the cortex known as the uncinate convolution and anterior 
part of the gyrus fornicatus. Peripheral stimulation* of this mechanism 
develops nerve impulses which, transmitted to the cortex, evoke the sensa- 
tions of odor. The specific physiologic stimulus is matter in the gaseous 
or vaporous state. 

For the appreciation of odorous particles the air must be drawn through 
the nasal fossae with a certain degree of velocity. If the particles are 
widely diffused in the air, they must be drawn not only more quickly but 
more forcibly into contact with the olfactory hairs, as in the act of sniffing, 



SENSE OF SIGHT 223 

the result of short energetic inspirations. To many substances the ol- 
factory apparatus is extremely sensitive. Thus, it has been shown that 
a particle of mercaptan the actual weight of which was calculated to be 
3^460.000,000 of a milligram gives rise to a distinct sensation. 

The Olfactory Sensations. — The sensations which arise in consequence 
of the excitation of the olfactory apparatus are very numerous and their 
classification is extremely difficult. For this reason it is customary to 
divide them into two groups: viz., agreeable and disagreeable, in accord- 
ance with the feelings they excite in the individual. As the olfactory 
sensations give rise to feelings rather than ideas, this sense plays in man a 
subordinate part in the acquisition of knowledge. In lower animals this 
sense is employed for the purpose of discovering and securing food, for 
detecting enemies and friends, and for sexual purposes. In land animals 
the entire olfactory apparatus is well developed and the sense keen; in 
some aquatic animals, as the dolphin, whale, and seal, the apparatus is 
poorly developed and the sense dull. 

THE SENSE OF SIGHT 

The physiologic mechanism involved in the sense of sight includes the 
eyeball, the optic nerve, the optic tracts, the thalamo-occipital tract or 
the optic radiation, and nerve-cells in the cuneus and adjacent gray matter. 
Peripheral stimulation of this mechanism develops nerve impulses which 
transmitted to the cortex evoke (1) the sensation of light and its different 
qualities — colors; (2) the perception of light and color under the form of 
pictures of external objects; and (3) in connection with the ocular muscles, 
the production of muscle sensations by which the size, distance, and direc- 
tion of objects may be judged. 

The specific physiologic stimulus to the terminal end-organ, the retina, 
is the impact of ether vibrations. In general, it may be said that, at 
least for the same color, the intensity of the objective vibration determines 
the intensity of the sensation. 

The Eyeball. — The eyeball, or organ of vision, is situated at the fore 
part of the orbital cavity and is supported by a cushion of fat; it is pro- 
tected from injury by the bony walls of the cavity, the lids, and the lashes, 
and is so situated as to permit of an extensive range of vision. The eyeball 
is loosely held in position by a fibrous membrane, the capsule of Tenon, 
which is attached on the one hand to the eyeball itself and on the other to 
the walls of the cavity. Thus suspended, the eyeball is capable of being 
moved in any direction by the contraction of the muscles attached to it. 



224 HUMAN PHYSIOLOGY 

Structure. — The eyeball is spheroid in shape and measures about twenty- 
four mm. in its anteroposterior diameter, and a little less in its transverse 
diameter. When viewed in profile, it is seen to consist of the segments 
of two spheres, of which the posterior is the larger, occupying five sixths, 
and the anterior the smaller, occupying one sixth, of the ball. 

The eye is made up of several membranes, concentrically arranged, 
within which are inclosed the refracting media essential to vision. 

The membranes, enumerating them from without inward, are as follows : 
the sclera and cornea, the choroid, iris and ciliary muscle, and the retina. 
The refracting media are the aqueous humor, the crystalline lens, and 
the vitreous humor. 

The Sclera and Cornea. — The sclera is the opaque fibrous membrane 
covering the posterior five sixths of the ball. It is composed of connec- 
tive tissue arranged in layers, which run both transversely and longitudin- 
ally; it is pierced posteriorly by the optic nerve about Jio of an inch inter- 
nal to the optic axis. The sclera, by its density, gives form to the eye and 
protects the delicate structures within it, and serves for the attachment 
of the muscles by which the ball is moved. 

The cornea is a transparent non-vascular membrane covering the ante- 
rior one sixth of the eyeball; It is nearly circular in shape and is continu- 
ous at the circumference with the sclera, from which it cannot be separated. 
The substance of the cornea is made up of thin layers of delicate, trans- 
parent fibrils of connective tissue, more or less united; between these 
layers are found a number of intercommunicating lymph-spaces, lined by 
endothelium, which are in connection with lymphatics. Leukocytes or 
lymph-corpuscles are often found in these spaces. At the junction of 
the cornea and sclera there is a circular groove, the canal of Schlemm. 

The Choroid, the Iris and the Ciliary Muscle. — These three structures 
together constitute the second or middle coat of the eyeball. 

The choroid is a dark brown membrane which extends forward nearly to 
the cornea, where it terminates in a series of folds, the ciliary processes. 
In its structure the choroid is highly vascular, consisting of both arteries 
and veins. Externally it is connected with the sclerotic by connective 
tissue; internally it is lined by a layer of hexagonal pigment cells, which, 
though usually classed as belonging to the choroid, is now known to belong, 
embryologically and physiologically, to the retina. 

The choroid with its contained blood-vessels bears an important rela- 
tion to the nutrition of the eye; it provides for the blood-supply and for 
drainage from the body of the eye, and presents a uniform and high tem- 
perature to the retina. 



SENSE OF SIGHT 225 

The iris is the circular variously colored membrane placed in the ante- 
rior portion of the eye just behind the cornea. It is perforated a little to 
the nasal side of the center by a circular opening, the pupil. The outer 
or circumferential border is connected with the cornea, ciliary muscle, and 
ciliary processes; the free inner edge forms the boundary of the pupil, the 
size of which is constantly changing. The framework of the iris is com- 
posed of connective-tissue blood-vessels, muscle-fibers and pigmented 
connective-tissue corpuscles. The anterior surface is covered with a layer 
of epithelial cells continuous with those covering the posterior surface of 
the cornea; the posterior surface is lined by a limiting membrane bearing 
pigment epithelial cells continuous with those of the choroid. The vari- 
ous colors which the iris assumes in different individuals depend upon the 
quantity and disposition of the pigment granules. 

The muscle-fibers of the iris, which are of the non-striated variety, are 
arranged in two sets — sphincter and dilatator. 

The sphincter pupillce is a circular, flat band of muscle-fibers surround- 
ing the pupil close to its posterior surface; by its contraction and relaxa- 
tion the pupil is diminished or increased in size. The dilatator pupillce con- 
sists of a thin layer of fibers arranged in a radiate manner; at the margin 
of the pupil they blend with those of the sphincter muscle, while at the 
outer border they arch to form a circular muscular layer. 

The ciliary muscle is a gray, circular band, consisting of unstriped 
muscle-fibers about 3^ of an inch long running from before backward. 
It is attached anteriorly to the inner surface of the sclera and cornea, 
and posteriorly to the choroid coat opposite the ciliary processes. At the 
anterior border of the radiating fibers and internally are found bundles of 
•circular muscle-fibers, constituting the annular muscle of Miiller. The 
ciliary muscle thus consists of two sets of fibers, a radiating and a circular, 
both of which are concerned in effecting a change in the convexity of the 
lens in the accommodation of the eye to near vision. 

The retina forms the internal coat of the eye. In the fresh state it is a 
delicate, transparent membrane of a pink color, but after death soon be- 
comes opaque; it extends forward almost to the ciliary processes, where it 
terminates in an indented border, the ora serrala. In the posterior part 
1 of the retina, at a point corresponding to the axis of vision, is a yellow spot, 
the macula lutea, which is somewhat oval in shape and tinged with yellow 
pigment. It presents in its center a depression, the fovea centralis, corre- 
. sponding to a decrease in thickness of the retina; about J-f of an inch to 
the inner side of the macula is the point of entrance of the optic nerves. 
The arteria centralis retince pierces the optic nerve near the sclerotic, runs 

15 



226 



HUMAN PHYSIOLOGY 



forward in its substance, and is distributed in the retina as far forward as 
the ciliary processes. 

The retina is remarkably complex consisting of ten distinct layers 
from without inward. For physiologic purposes they may be resolved 
into three — viz.: 

i. The layer of visual cells, the rods and cones. 

2. The layer of bipolar cells. 

3. The layer of ganglionic cell. Fig. 23. 
The number of optic nerve-fibers in the retina is 

estimated to be about 800,000, and for each fiber 
there are about seven cones, one hundred rods, and 
seven pigment cells. The points of the rods and 
cones are directed toward the choroid, or away from 
the entering light, and dip into the pigment layer. 
They, with the pigment layer, are the intermediat- 
ing elements in the change of the ethereal vibra- 
tions into nerve force; out of these nerve vibrations 
the brain fashions the sensations of light, form, and 
color. 

The Refracting Media. — The vitreous humor, 
which supports the retina, is the largest of the re- 
fracting media; it is globular in form and constitutes 
about four-fifths of the ball, it is hollowed out ante- 
riorly for the reception of the crystalline lens. The 
IG * 23 Cells. T l N outer surface of the vitreous is covered by a delicate, 
5', z'. Visual cells transparent membrane, termed the hyaloid membrane, 
t W ermini h tiL Pe 5! P Rods! which serves to maintain its globular form. 

z. Cones, b. Bipolar The aqueous humor, found in the anterior chamber 

cells, g. Ganglion cells . ' ■ . 

from which arise the of the eye, is a clear alkaline fluid, having a specific 

nerv!. ° f the ° Pti ° gravity of 1003-1009. It is secreted most probably 
by the blood-vessels of the iris and ciliary processes. 
It passes from the interior of the eye, through the canal of Schlemm and 
the meshes at the base of the iris, into the lymph vessels and thus in- 
creased ocular tension is prevented. 

The crystalline lens, inclosed within its capsule, is a transparent biconvex 
body, situated just behind the iris and resting in the depression in the 
anterior part of the vitreous. The two convexities are not quite alike, 
the curvature of the posterior surface being slightly greater than that of 
the anterior. The lens measures about }& of an inch in the transverse 
diameter and 3^ of an inch in the anteroposterior diameter. 

The suspensory ligament, by which the lens is held in position, is a firm, 




SENSE OF SIGHT 



227 







Pig. 24. — Sclerotic Coat Removed to Show Choroid Ciliary Muscle, and 

Nerves. — (Holden.) 
a. Sclerotic coat. b. Veins of the choroid, c, Ciliary nerves, d. Veins of the 
choroid, e. Ciliary muscle. /. Iris. 




Fig. 25. — Diagram of a Vertical Section of the Eye. — {Holden.) 
t. Anterior chamber filled with aaueous humor. 2. Posterior chamber. 3- Canal 
of Petit, a. Hyaloid membrane. 0. Retina (dotted line), c. Choroid coat (black 
line), d. Sclerotic ,'coat. e. Cornea. /. Iris. g. Ciliary processes, h. Canal of 
Schlemn or Fontana. »', Ciliary muscle. 



228 HUMAN PHYSIOLOGY 

transparent membrane, united to the ciliary processes. A short distance 
beyond its origin it splits into two layers, the anterior of which is inserted 
into the capsule of the lens and blends with it; the posterior, passing in- 
ward behind the lens, becomes united to its capsule. The anterior layer 
presents a series of foldings, zone of Zinn, which are inserted into the inter- 
vals of the folds of the ciliary processes. The triangular space between 
the two layers is the canal of Petit. 

Blood-vessels and Nerves. — The structures composing the eyeball are 
supplied with blood by the long and short ciliary arteries, branches of the 
opthalmic; they pierce the sclerotic at various points and are ultimately 
distributed to all tissues within the ball. 

The nerves distributed to the non-striated muscles of the eyeball — the 
ciliary muscle and the sphincter muscle of the iris — are postganglionic 
fibers coming from the ciliary or ophthalmic ganglion; distributed to this 
ganglion are preganglionic fibers coming from the central nerve system 
through the oculo-motor nerve. The nerves distributed to the dilatator 
muscle of the iris and to the blood-vessels are postganglionic fibers com- 
ing from the superior cervical ganglion; distributed to this ganglion are 
preganglionic fibers coming from the central nerve system through the 
upper thoracic nerves and the cervical cord of the sympathetic. Sensory 
nerves are derived from the trigeminal. The relationship of the structures 
composing the eyeball is shown in Figs. 24, 25. 

THE PHYSIOLOGY OF VISION 

The Retinal Image. — The general function of the eye is the formation 
of images of external objects on the free ends of the percipient elements of 
the retina, the rods and cones. The existence of an image on the retina 
can be readily seen in the excised eye of an albino rabbit, when placed 
between a lighted candle and the eye of an observer. Its presence in the 
human eye can be demonstrated with the ophthalmoscope. It is this 
image, composed of focal points of luminous rays, that stimulate the rods 
and cones, which is the basis of our sight perceptions, and out of which the 
mind constructs space relations of external objects. Whatever the dis- 
tance, the image is generally smaller than the object; it is also reversed, 
the upper part of the object becoming the lower part of the image, and the 
right side of the object the left side of the image. 

The Dioptric or Refracting Apparatus. — The formation of an image is 
made possible by the introduction of a complex refracting apparatus con- 
sisting of the cornea, aqueous humor, lens, and vitreous humor. Without 



riJYSIOLOGY OF VISION 229 

these agencies the ether vibrations would give rise only to a sensation of 
diffused luminosity. Rays of light emanating from any one point arriv- 
ing at the eye must traverse successively the different refracting media. 
In their passage from one to the other, they undergo at their surfaces 
changes in direction before they are finally converged to a focal point on 
the retina. 

Inasmuch as the two surfaces of the cornea are parallel and its refractive 
power practically the same as the aqueous humor, the media may be 
reduced to three — viz.: 

1. Cornea and aqueous humor. 

2. The lens. 

3. The vitreous humor. 

The refracting surfaces may also be reduced to three — viz. : 

1. Anterior surface of the cornea. 

2. Anterior surface of lens. 

3. Posterior surface of lens. 

The refraction effected by the cornea is very great, owing to the passage 
of the light from the air into a comparatively dense medium, and is suffi- 
cient of itself to bring parallel rays of light to a focus about ten millimeters 
behind the retina. This would be the condition in an eye in which the 
lens was congenitally absent or after removal by surgical procedures. 
Perfect vision requires, however, that the convergence of the light shall 
be great enough to allow the image to fall upon the retina. This is accom- 
plished in part by the crystalline lens, a body denser than the cornea and 
possessing a higher refractive power. After passing through the lens the 
rays of light if continued would come to a focus about 6.5 mm. behind the 
retina. On passing from the lens into the vitreous — i.e., from a denser 
into a rarer medium — the rays are once more converged and to an extent 
sufficient to focalize them on the retina. The function of the cornea and 
lens is to focalize the rays with the production of an image. 

The Visual Angle. — The visual angle is defined as the angle formed by 
the intersection of two lines drawn from the extremities of an object to 
the nodal point of the eye which lies near the posterior surface of the lens 
about 15.5 millimeters from the retina. Beyond the nodal point, however, 
the lines again diverge and form an inverted or reversed image of the ob- 
ject on the retina. The size of the visual angle increases with the near- 
ness and deer ith the remoteness of the object; the retinal image 
pondingly increases and decreases in size. 

The Size of the Retinal Image.— The size of the retinal image depends 
upon the visual angle, which in turn depends upon the size of the object 



230 HUMAN PHYSIOLOGY 

and its distance from the eye. At a distance of 15.2596 meters the image 
of an object one meter high would be one millimeter, or a thousand times 
smaller than the object. 

The size of the image may be calculated from the following equation. 
The size of the object is to the size of the image, as the distance of the 
object from the nodal point, is to the distance of the nodal point from the 
retina. (The distance of the nodal point from the anterior surface of 
the cornea is 7.3 mm.). 

Accommodation. — By accommodation is understood the power which 
the eye possesses of adjusting itself to vision at different distances. In a 
normal or emmetropic eye parallel rays of light are brought to a focus on 
the retina; but divergent rays — that is, rays coming from a near luminous 
point — will be brought to a focus behind the retina, provided the refrac- 
tive media remain the same; as a result, vision would be indistinct, from 
the formation of diffusion circles. It is impossible to see distinctly, there- 
fore, a near and a distant object at the same time. We must alternately 
direct the vision from one to the other. A normal eye does not require 
adjusting for parallel rays; but for divergent rays a change in the eye is 
necessitated; this is termed accommodation. In the accommodation for 
near vision the lens becomes more convex, particularly on its anterior 
surface. The increase in convexity augments its refractive power; the 
greater the degree of divergence of the rays previous to entering the 
eye, the greater the increase of convexity of the lens and convergence of 
the rays after passing through it. By this alteration in the shape of the 
lens we are enabled to focus light rays coming from, and to see distinctly, 
near as well as distanct objects. 

Function of the Ciliary Muscle. — Though it is admitted that the change 
in the convexity of the lens is caused by the contraction of the ciliary 
muscle and the relaxation of the suspensory ligament, the exact manner 
in which it does so is not understood. When the eye is in repose, as in 
distant vision, the suspensory ligament is tense, and the lens possesses that 
degree of curvature necessary for focusing parallel rays. In the voluntary 
efforts to accommodate the eye for near vision, the ciliary muscle contracts, 
the suspensory ligament relaxes, and the lens, inherently elastic, bulges 
forward and once again focuses the rays upon the retina. It is, therefore, 
termed the muscle of accommodation, and by its alternate contraction 
and relaxation the lens is rendered more or less convex, according to the 
requirements for near and distant vision. 

Range of Accommodation. — Parallel rays coming from a luminous point 
distant not less than 200 feet do not require adjustment; from this point 



PHYSIOLOGY OF VISION 231 

up to infinity no accommodation is required for perfect vision. This is 
termed the punctum remotum, and indicates the distance to which an 
object may be removed and yet distinctly seen. If the object be brought 
nearer to the eye than 200 feet, the accommodative power must come into 
play; the nearer the object, the more energetic must be the contraction 
of the ciliary muscle and the consequent increase in the convexity of the 
lens. At a distance of five inches, however, the power of accommodation 
reaches its maximum; this is termed the punctum proximum, and indicates 
the nearest point at which an object may be seen distinctly. The dis- 
tance between these two points is the range of accommodation. 

The Function of the Iris. — The iris plays the part of a diaphragm, and 
by means of its central aperture the pupil regulates the quantity of 
light entering the interior of the eye; by preventing rays from passing 
through the margin of the lens it diminishes spheric aberration. The size 
of the pupil depends upon the relative degree of contraction of the circular 
and radiating fibers; the variations in size of the pupil from variations 
in the degree of contraction depend upon different intensities of light. If 
the light be intense, the circular fibers contract, and diminish the size of 
the pupil; if the light diminishes in intensity, the circular fibers relax and 
the pupil enlarges. 

Point of Most Distinct Vision. — While all portions of the retina are 
sensitive to light, their sensibility varies within wide limits. At the 
macula lutea, and more especially in its most central depression, the fovea, 
where the retinal elements are reduced practically to the layer of rods and 
cones, the sensibility reaches its maximum. It is at this point that the 
image is found when vision is most distinct. The macula and fovea are 
always in the line of direct vision. From the macula toward the periphery 
of the retina there is a gradual diminution in sensibility, and a corre- 
sponding decline in the distinctness of vision. In those portions of the 
retina lying outside the macula, the indistinctness of vision depends not 
only on diminished sensibility, but also upon inaccurate focusing of the 

Blind Spot. — Although the optic nerve transmits the impulses excited 
in the retina by the ethereal vibration, the nerve-fibers themselves are in- 
itive to light. At the point of entrance of the optic nerve, owing to 
the absence of the rods and cones, the rays of light make no impression. 
This is the blind spot. As this spot is not in the line of vision, no dark 
point is ordinarily observed in the field of vision — the circular space before 
a fixed eye within which reflections of objects are perceptible. 

The rods and cones are the most sensitive portions of the retina. A ray 



232 HUMAN PHYSIOLOGY 

of light entering the eye passes entirely through the various layers of the 
retina, and is arrested only upon reaching the pigmentary' epithelium in 
which the rods and cones are embedded. As to the manner in which the 
objective stimuli — light and color, so called — are transformed into nerve 
impulses, but little is known. It is probable that the ethereal vibrations 
are transformed into heat, which excites the rods and cones. These, act- 
ing as highly specialized end organs of the optic nerve, start the impulses 
on their way to the brain, where the seeing process takes place. As to 
the relative function of the rods and cones, it has been suggested, from the 
study of the facts of comparative anatomy, that the rods are impressed 
only by differences in the intensity of light, while the cones, in addition 
are impressed by qualitative differences or color. 

The Eyeball a Living Camera Obscura. — The eyeball may be compared 
in a general way to a camera obscura. The anatomic arrangement of its 
structures reveals many points of similarity. The sclerotic and choroid 
may be compared with the walls of the chamber; the combined refractive 
media, cornea, aqueous humor, lens, and vitreous humor, to the lens for 
focusing the rays of light; the retina, to the sensitive plate receiving the 
image formed at the focal point; the iris, to the diaphragm, which, by 
cutting off the marginal rays, prevents spheric aberration and at the same 
time regulates the amount of light entering the eye; the ciliary muscle, 
to the adjusting screw, by which distinct images are thrown upon the 
retina in spite of varying distances of the object from which the light rays 
emanate. 

OPTIC DEFECTS 

Presbyopia. — Presbyopia may be defined as a condition of the normal 
eye in which the accommodation has become so reduced by age that read- 
ing has become impossible at ordinary distances. As age advances the 
lens gradually loses its elasticity and hence its power to increase in con- 
vexity and thickness to the same extent as in earlier life, in response to 
efforts of accommodation. The refractive power is, thereby, lessened 
and the eye is no longer able to see distinctly at the normal reading dis- 
tances, viz. : 22 to 28 cm. Rays of light emanating from a luminous point 
at the normal reading distances are less and less converged on the retina 
and hence the diffusion circles increase in size. The near point, the point 
from which divergent rays can be focalized, therefore advances toward 
the far point, or recedes from the individual. The range of accommoda- 
tion is, thereby, diminished. 



OPTIC DEFECTS 27,7, 

Myopia. — Myopia may be defined as a condition of the eye character- 
ized by an increase in the antero-posterior diameter or by a hypernormal 
refracting power of the lens. The former is the usual condition. In 
either case parallel rays of light which enter the eye are brought to a focus 
in front of the retina after which they diverge and give rise to diffusion 
circles and indistinctness of vision. Divergent rays, however, which enter 
the eye are focalized as usual on the retina even in its new position. The 
distant point, the punctum remotum, is always at a finite distance, but 
approaches the eye as the myopia increases. The near point is usually 
much nearer the eye than 20 cm. For this reason the condition is termed 
near sight. 

Hypermetropia. — Hypermetropia may be defined as a condition of the 
eye characterized by decrease of the normal antero-posterior diameter or 
by a subnormal refracting power of the lens. The former is the usual con- 
dition. In either case parallel rays of light which enter the eye are, there- 
fore, not brought to a focus when the accommodation is suspended. Fall- 
ing on the retina previous to focalization, they give rise to diffusion- 
circles and indistinctness of the image. As no object can be seen distinctly 
no matter how remote, there is no positive far point. The near point is 
abnormally distant — sometimes as far as 200 cm. For this reason the 
condition is termed far sight. A hypermetropic eye without accommoda- 
tive effort can focalize only converging rays on the retina. 

Astigmatism. — Astigmatism may be defined as a condition of the eye 
characterized by an inequality of curvature of its refracting surfaces in 
consequence of which not all of the rays coming from a single point are 
brought to the same focus. The inequality may be either in the cornea 
or lens, or both, though usually in the cornea. 

In the normal cornea the radius of curvature in the vertical meridian 
is a trifle shorter, 7.6 mm., than that of the horizontal, 7.8 mm., and 
hence its focal distance is slightly shorter. The difference, however, in 
the focal distances is so slight that the error in the formation of the 
image is scarcely noticeable. A transverse section of a cone of light com- 
ing from the cornea is practically a circle. If, however, the vertical 
curvature exceeds the normal to any marked extent, the rays passing in 
the vertical plane will be more sharply refracted and brought to a focus 
much sooner than the ray through the horizontal plane. The 

result will be that the cone of light will be no longer circular, but more 
or less elliptic. Though the vertical plane has usually the sharper curva- 
ture, it not infrequently happens that the reverse is true. For the reason 



234 HUMAN PHYSIOLOGY 

that the rays from one point do not all come to the same focus or point, 
the condition is termed astigmatism. 

Movements of the Eyeball. — The almost spheric eyeball lies in the 
correspondingly shaped cavity of the orbit, like a ball placed in a socket, 
and is capable of being rotated to a considerable extent by the six muscles 
which are attached to it. These muscles are the superior and inferior 
recti, the external and internal recti, and the superior and inferior obliqui. 
The four recti muscles arise from the apex of the orbit cavity, from which 
point they pass forward to be inserted into the sclera about 7 to 8 mm. 
from the corneal border. The superior oblique muscle having a similar 
origin passes forward to the upper and inner angle of the orbit cavity, 
at which point its tendon passes through a cartilaginous pulley, after 
which it is reflected backward to be inserted into the superior surface of 
the sclera about 16 mm. behind the corneal border. The inferior oblique 
muscle arises from the inner and inferior angle of the orbit cavity. It 
then passes outward, upward, and backward, to be inserted into the 
upper, posterior, and temporal portion of the sclera about 4 or 5 mm. 
from the optic nerve entrance. 

The superior and inferior recti muscles, forming one pair, move the eye 
around a horizontal axis which intersects the median plane of the body 
in front of the eyes at an angle of 63 degrees; the external and internal 
recti muscles, forming a second pair, move the eye around a vertical 
axis; the superior and inferior oblique muscles forming the third pair 
rotate the globe around a horizontal axis which cuts the median plane of 
the body behind the eyes at an angle of 39 degrees. Thus it is that each 
muscle moves the eye as follows, the movement for practical purposes 
being referred to the cornea: The rectus externus draws the cornea 
simply to the temporal side, the rectus internus simply to the nose; the 
superior rectus displaces the cornea upward, slightly inward, and turns 
the upper part toward the nose (medial torsion) ; the inferior rectus moves 
the cornea downward, slightly inward, and twists the upper part away 
from the nose (lateral torsion); the superior oblique displaces the cornea 
downward, slightly outward, and produces medial torsion; while the 
inferior oblique moves the cornea upward, slightly outward, and produces 
lateral torsion. These facts show that for certain movements of the 
eye at least three muscles are necessary. 

THE SENSE OF HEARING 

The physiologic mechanism involved in the sense of hearing includes 
the ear, the acoustic nerve, the acoustic tract (the lateral fillet or lem- 



SENSE OF HEARING 235 

niscus), the acoustic radiation, and nerve-cells in the thorax of the 
temporal lobe. 

Peripheral stimulation of this mechanism develops nerve impulses 
which, transmitted to the cerebral cortex, evoke the sensation of sound 
and its varying qualities — intensity, pitch, and timbre. 

The specific physiologic stimulus to the terminal organ, the organ of 
Corti, is the impact of atmospheric pulsations of varying energy and 
rapidity. 

The ear, or organ of hearing, is lodged within the petrous portion of 
the temporal bone. It may be, for convenience of description, divided 
into three portions — viz.: 

1. The external ear. 

2. The middle ear. 

3. The internal ear or labyrinth. 

The External Ear. — The external ear consists of the pinna, or auricle, 
and the external auditory canal. The pinna consists of a thin layer of 
cartilage, presenting a series of elevations and depressions; it is attached 

1 by fibrous tissue to the outer bony edge of the auditory canal; it is covered 
by a layer of integument continuous with that covering the side of the 
head. The general shape of the pinna is concave, and presents, a little 
below the center, a deep depression — the concha. The external auditory 
canal extends from the concha inward for a distance of about 1% inches. 
It is directed somewhat forward and upward, passing over a convexity 
of bone, and then dips downward to its termination; it is composed of 
both bone and cartilage, and is lined with a reflection of the skin covering 
the pinna. At the external portion of the canal the skin contains a number 
of tubular glands — the rem mi nous glands — which in their conformation 
:nble the perspiratory glands. They secrete the cerumen, or ear-wax. 
The Middle Ear. — The middle ear, or tympanum, is an irregularly 
shaped cavity hollowed out of the temporal bone and situated between 
the external ear and the labyrinth. It is narrow from side to side, but 
relatively long in its vertical and anteroposterior diameters; it is separated 
from the external auditory canal by a membrane — the membrana tympani; 
from the internal ear il ated by an osseo-membranous partition, 

which forms a common wall for both cavities. The middle ear communi- 
cates posteriorly with the m lis; anteriorly with the nasopharynx, 
by means of the Eustachian tube. The interior of this cavity is lined by 

j mucous membrane continuous with that lining the pharynx (Fig. 26). 
The Membrana Tympani. — The membrana tympani is a thin, trans- 
lucent, nearly circular membrane, measuring about £5 of an inch in di- 



236 



HUMAN PHYSIOLOGY 



ameter, placed at the inner termination of the external auditory canal. 
The membrane is inclosed within a ring of bone, which in the fetal condi- 
tion can be easily removed, but in the adult condition becomes consoli- 
dated with the surrounding bone. The membrana tympani consists 
primarily of a layer of fibrous tissue, arranged both circularly and radially, 
and forms the membrana propria; externally it is covered by a thin layer 
of skin continuous with that lining the auditory canal; internally it is 




Fig. 26. — Tympanum and Auditory Ossicles (Left) Magnified. 
A.G. External meatus. M. Membrana tympani, which is attached to the handle 
of the malleus, n, and near it the short process, p. h. Head of the malleus, a. 
Incus; K. its short process, with its ligament; I. long process, s. Sylvian ossicle. 
S. Stapes. Ax, Ax, is the axis of rotation of the ossicles; it is shown in perspective 
and must be imagined to penetrate the plane of the paper, t. Line of traction of 
the tensor tympani. The other arrows indicate the movement of the ossicles when 
the tensor contracts. 



covered by a thin mucous membrane. The tympanic membrane is 
placed obliquely at the bottom of the auditory canal, inclining at an 
angle of forty-five degrees, being directed from behind and above down- 
ward and inward. On its external surface this membrane presents a 
funnel-shaped depression, the sides of which are somewhat convex. 

The Ear Bones. — Running across the tympanic cavity and forming 
an irregular line of joined levers is a chain of bones which articulate with 



SENSE OF HEARING 237 

one another at their extremities. They are known as the malleus, incus, 
and stapes. 

The form and position of these bones are shown in figure 36. 

The malleus consists of a head, neck, and handle, of which the latter 
is attached to the inner surface of the membrana tympani; the incus, 
or anvil bone, presents a concave, articular surface, which receives the 
head of the malleus; the stapes, or stirrup bone, articulates externally 
with the long process of the incus, and internally, by its oval base, with 
the edges of the foramen ovale. 

The Tensor Tympani. — The tensor tympani muscle consists of a fleshy, 
tapering portion, J^ of an inch in length, which terminates in a slender 
tendon; it arises from the cartilaginous portion of the Eustachian tube 
and the adjacent surface of the sphenoid bone. From this origin the 
muscle passes nearly horizontally backward to the tympanic cavity; just 
opposite to the fenestra ovalis its tendon bends at a right angle over the 
processus cochleariformis, and then passes outward across the cavity, to 
be inserted into the angle of the malleus near the neck. 

The Stapedius Muscle. — The stapedius muscle emerges from the cavity 
of a pyramid of bone projecting from the posterior wall of the tympanum; 
the tendon passes forward, and is inserted into the neck of the stapes bone, 
posteriorly, near its point of articulation with the incus. 

The Eustachian Tube. — The Eustachian tube, by means of which a free 
communication is established between the middle ear and the pharynx, 
is partly bony and partly cartilaginous in structure. It measures about 
1 3^ inches in length; commencing at its opening into the nasopharynx, 
it passes upward and outward to the spine of the sphenoid bone, at which 
point it becomes somewhat contracted; the tube then dilates as it passes 
backward into the middle-ear cavity; it is lined by mucous membrane, 
which is continued into the middle ear and mastoid cells. 

The Function of the Ear. — The function of the ear, as a whole, is the 
reception and transmission of aerial vibrations to the terminal organs 
concealed within the internal ear, and which are connected with the 
auditory nerve-fibers. The excitation of these end organs caused by the 
impact of the vibration arouses in the auditory nerve impulses which are 
then transmitted to the brain, where the hearing process takes place. 
In order to appreciate the functions of the individual parts of the ear, a 
few of the characteristics of sound waves must be kept in mind. 

Sound Waves. — All sounds are caused by vibrations in the atmosphere 
which have been communicated to it by vibrating elastic bodies, such as 



238 HUMAN PHYSIOLOGY 

membranes, strings, rods, etc. These vibrating bodies produce in the 
air a to-and-fro movement of its particles, resulting in a series of alternate 
condensations and rarefactions, which are propagated in all directions. 
A complete oscillation of a particle of air forward and backward consti- 
tutes a sound wave. Musical sounds are caused by a succession of regular 
waves, which follow one another with a certain rapidity. Noises are 
caused by the impact of a series of irregular waves. 

All sound waves possess intensity, pitch, and equality. The intensity, 
or loudness, of a sound depends upon the amplitude of its vibrations or 
on the extent of its excursion. The pitch depends upon the number of 
vibrations which affect the auditory nerve in a second of time; the pitch 
of the note C, the first below the leger line of the musical scale, is caused 
by 256 vibrations a second; the pitch of the same note an octave higher 
is caused by 512 vibrations a second. If the vibrations are too few a 
second, they fail to be perceived as a continuous sound; the minimum 
number of vibrations capable of producing a sound has been fixed at 
sixteen a second; the highest pitched musical note capable of being heard 
has been shown to be due to 38,000 vibrations a second. In the ascent 
of the musical scales there is, therefore, a gradual increase in the number 
of vibrations and a gradual increase in the pitch of the sounds. Between 
the two extreme limits lies the range of audibility, which embraces eleven 
octaves, of which seven are employed in the musical scale. 

The quality of sound depends upon a combination of the fundamental 
vibration with certain secondary vibrations of subdivisions of the vibrat- 
ing body. These so-called over-tones vary in intensity and pitch, and 
by modifying the form of the primary wave produce that which is termed 
the quality of sound. 

Function of the Pinna and External Auditory Canal. — In those animals 
possessing movable ears the pinna plays an important part in the collection 
of sound waves. In man, in whom the capability of moving the pinna 
has been lost, it is doubtful if it is at all necessary for hearing. Never- 
theless an individual with dull hearing may have the perception of sound 
increased by placing the pinna at an angle of 45 degrees to the side of the 
head. The external auditory canal transmits the sonorous vibrations to 
the tympanic membrane. Owing to the obliquity of this canal it has 
been supposed that the waves, concentrated at the concha, undergo a 
series of reflections on their way to the tympanic membrane, and, owing 
to the position of this membrane, strike it almost perpendicularly. 

Function of the Tympanic Membrane. — The function of the tympanic 
membrane appears to be in the reception of sound vibrations by being 



SENSE OF HEARING 239 

thrown by them into reciprocal vibrations which correspond in intensity 
and amplitude. That this membrane actually reproduces all vibrations 
within the range of audibility has been experimentally demonstrated. 
The membrane not being fixed, so tar as its tension is concerned, does not 
possess a fixed fundamental note, like a stationary fixed membrane, and is, 
therefore, just as well adapted for the reception of one set of vibrations as 
for another. This is made possible by variations in its tension in ac- 
cordance with the pitch of the sounds. In the absence of all sound the 
membrane is in a condition of relaxation; with the advent of sound 
waves possessing a gradual increase of pitch, as in the ascent of the 
music scale, the tension of the tympanic membrane is gradually in- 
creased until its maximum tension is reached at the upper limit of the 
range of audibility. By this change in tension certain tones become 
perceptible and distinct, while others become indistinct and inaudible. 

Function of the Tensor Tympani Muscle. — The function of this 
muscle is, as its name indicates, to increase the tension of the membrane 
in accordance with the pitch of the sound wave. The tension of this mus- 
cle playing over the processus cochleariformis and attached at also a 
right angle to the handle of the malleus mil, when the muscle contracts, 
pull the handle inward, increase the convexity of the membrane, and at 
the same time increase its tension; with the relaxation of this muscle, the 
handle of the malleus passes outward and the tension is diminished. The 
contractions of the tensor muscle are reflex in character and excited by 
nerve impulses reaching it through the small petrosal nerve and otic 
ganglion. The number of nerve stimuli passing to the muscle and deter- 
mining the degree of contraction will depend upon the pitch of the sound 
wave and the subsequent excitation of the auditory nerve. The tensor 
tympani muscle may be regarded as an accommodative apparatus by which 
the tympanic membrane is so adjusted as to enable it to receive vibra- 
tions of varying degrees of pitch. 

Function of the Ear Bones. — The function of the chain of bones is to 
transmit the sound wave across the tympanic cavity to the internal ear. 
The first of these bones, the malleus, being attached to the tympanic 
membrane, will take up the vibrations much more readily than if no mem- 
brane intervened. Owing to the character of the articulations when the 
handle of the malleus is drawn inward, the position of the bones 1 
changed that they form practically a solid rod, and are therefore much 
better adapted for the transmission of molecular vibrations than if the 
articulations remained loose. As the stapes bone, is somewhat shorter 
than the malleus, its vibrations are slighter than those of the tympanic 



240 HUMAN PHYSIOLOGY 

membrane, and by this arrangement the amplitude of the vibrations is 
diminished, but their force increased. 

The function of the stapedius muscle is, according to Henle, to fix 
the stapes bone so as to prevent too great a movement from being com- 
municated to it from the incus and transmitted' to the perilymph. It 
may be looked upon, therefore, as a protective muscle. 

The Function of the Eustachian Tube. — The function of the Eustachian 
tube is to maintain a free communication between the cavity of the middle 
ear and the nasophyarnx. The pressure of air within and without the 
ear is thus equalized, and the vibrations of the tympanic membrane are 
permitted to attain their maximum, one of the conditions essential 
for the reception of sound waves. The impairment in the acuteness of 
hearing which is caused by an unequal pressure of the air in the middle 
ear can be shown — 

i. By closing the mouth and nose and forcing air from the lungs through 
the Eustachian tube into the ear, producing an increase in pressure. 

2. By closing the nose and mouth, and making efforts at deglutition 
which withdraws the air from the ear and diminishes its pressure. 

In both instances the free vibrations of the tympanic membrane are 
interfered with. The pharyngeal orifice of the Eustachian tube is opened 
by the action of certain of the muscles of deglutition — viz., the levator 
palati, the tensor palati, and the palato-pharyngei muscles. 

The Internal Ear. — The internal ear, or labyrinth, is located in the 
petrous portion of the temporal bone, and consists of an osseous and a 
membrane portion. 

The osseous labyrinth is divisible into three parts — viz., the vestibule, 
the semicircular canals, and the cochlea. 

The Vestibule is a small triangular-shaped cavity between the semi- 
circular canals and the cochlea. It is separated from the cavity of the 
middle ear by an osseous partition which presents near its center an 
oval opening, the foramen ovale. In the living condition this opening 
is closed by the base of the stapes bone, which is held in position by an 
annular ligament. The inner wall presents a number of openings for 
the passage of nerve-fibers. 

The Semi-circular canals are three in number, and named from their 
position, the superior vertical, the posterior vertical and the horizontal. 
These canals are at right angles one to the other and open by five orifices 
into the vestibule, one of the orifices, however, being common to two 
of the canals. Each canal near the vestibular orifice is enlarged to al- 






SENSE OF HEARING 241 

most twice the size of the rest of the canal, forming what lias been termed 
the ampulla. 

The Cochlea forms the anterior part of the internal ear. It is a gradually 
tapering canal, about 1 1 2 inches in length, which winds spirally around a 
central axis, the modiolus, two and one half times. The interior of the 
cochlea is partly divided into two passages by a thin plate of bone, the 
lamina osseous spiralis, which projects from the central axis two thirds 
of the way across the canal. These passages are termed the scala ves- 
tibuli and the scala tympani, horn their communication with the vestibule 
and tympanum. The scala tympani communicates with the middle 
ear through the foramen rotundum, which, in the natural condition, is 
closed by the second membrana tympani; superiorly they are united 
by an opening, the helicotrema. 

The whole interior of the labyrinth, the vestibule, the semicircular 
canals, and the scala of the cochlea, contains a clear, limpid fluid, the 
perilymph. 

The membranous labyrinth corresponds to the osseous labyrinth 
with respect to form, though it is somewhat smaller in size. 

The vestibular portion consists of two small sacs, the utricle and the 
saccule. 

The semicircular canals communicate with the utricle in the same 
manner as the bony canals communicate with the vestibule. The saccule 
communicates with the membranous cochlea by the canalis reuniens. 
In the interior of the utricle and saccule, at the entrance of the auditory 
nerve, are small masses of carbonate of lime crystals, constituting the 
otoliths. Their function is unknown. 

The membranous cochlea is a closed tube, commencing by a blind 
extremity at the first turn of the cochlea, and terminating at its apex 
by a blind extremity also. It is situated between the edge of the osseous 
lamina spiralis and the outer wall of the bony cochlea, and follows it 
in its turns around the modiolus. 

A transverse section of the cochlea shows that it is divided into two 
portions by the osseous lamina and the basilar membrane: 

1. The scala irstihuli. bounded by the periosteum and membrane of 
Reissner. 

2. The scala tympan'ui, occupying the inferior portion, and bounded 
above by the septum, composed of the osseous lamina and the membrana 
basilaris. 

The true membranous can>il i^ situated between the membrane of 
Reissner and the basilar membrane. It is triangular in shape, but is 
16 



242 HUMAN PHYSIOLOGY 

partly divided into a triangular portion and a quadrilateral portion by the 
tectorial membrane. 

The Organ of Corti. — The organ of Corti is situated in the quad- 
rilateral portion of the canal, and consists of pillars of rods of the consis- 
tence of cartilage. They are arranged in two rows — the one internal, 
the other external; these rods rest upon the basilar membrane; their 
bases are separated from one another, but their upper extremities are 
united, forming an arcade. In the internal row it is estimated there are 
about 3,500 and in the external row about 5,200 of these rods. 

On the inner side of the internal row is a single layer of elongated hair- 
cells; on the outer surface of the external row are three such layers of hair- 
cells. Nothing definite is known as to their function. 

The endolymph occupies the interior of the utricle, saccule, and mem- 
branous canals, and bathes the structures in the interior of the membran- 
ous cochlea throughout its entire extent. 

The Auditory Nerve. — The auditory nerve at the bottom of the inter- 
nal auditory meatus divides into — 

1. A vestibular branch, which is distributed to the utricle and to the 
semicircular canals. 

2. A cochlear branch, which passes into the central axis at its base and 
ascends to its apex; as it ascends, fibers are given off, which pass between 
the plates of the osseous lamina, to be ultimately connected with the 
organ of Corti. 

The Function of the Semicircular Canals. — The function of the semi- 
circular canals appears to be to assist in maintaining the equilibrium of 
the body; destruction of the vertical canal is followed by an oscillation of 
the head upward and downward; destruction of the horizontal canal is 
followed by oscillations from left to right. When the canals are injured 
on both sides, the animal loses the power of maintaining equilibrium upon 
making muscular movements. From these facts it is apparent that they 
are among the peripheral sense-organs, the physiologic action of which is 
the development of nerve impulses, which when transmitted to the brain 
assist the equilibratory mechanism to maintain the equilibrium of the 
body, both in the standing position and in the various modes of progres- 
sion. The character of the stimulus, however, and the manner in which 
it acts on the specialized portion of the sense-organs (the hair-cells) is 
not entirely clear. 

The Functions of the Cochlea. — The cochlea is the portion of the in- 
ternal ear which is concerned in the perception of tones. The arrange- 



PHONATION — ARTICULATE SPEECH 243 

ment of the histologic elements of the organ of Corti indicates that they 
in some way respond to the vibrations of varying frequency and form, and 
through the development of nerve impulses, evoke the sensations of pitch 
and quality. The manner in which this is accomplished is largely a mat- 
ter of speculation. 

Function of the Cochlea. — It is regarded as possessing the power of 
appreciating the quality of pitch and the shades of different musical tones. 
The elements of the organ of Corti are analogous, in some respects, to a 
musical instrument, and are supposed, by Helmholtz, to be tuned so as 
to vibrate in unison with the different tones conveyed to the internal ear. 

Summary. — The waves of sound are gathered together by the pinna 
and external auditory meatus, and conveyed to the membrana tympani. 
This membrane, made tense or lax by the action of the tensor tympani 
muscle, is enabled to receive sound waves of either high or low pitch. 
The vibrations are conducted across the middle ear by a chain of bones to 
the foramen ovale, and by the column of air of the tympanum to the fora- 
men rotundum, which is closed by the second membrana tympani, the 
pressure of the air in the tympanum being regulated by the Eustachian 
tube. 

The internal ear finally receives the vibrations, which excite vibrations 
successively in the perilymph, the walls of the membranous labyrinth, the 
endolymph, and, lastly, the terminal filaments of the auditory nerve, by 
which they are conveyed to the brain and evoke in the cortical cells the 
sensations of sound. 

PHONATION— ARTICULATE SPEECH 

Phonation, the emission of vocal sounds, is accomplished by the vibra- 
tion of two elastic membranes which cross the lumen of the larynx antero- 
posterior^ and which are thrown into vibration by a blast of air from the 
lungs. 

Articulate speech is a modification of the vocal sounds or the voice pro- 
duced by the teeth and the muscles of the lips and tongue and is employed 
for the expression of ideas. 

The larynx, the organ of the voice, is situated in the fore part of the neck, 
occupying the space between the hyoid bone and the upper extremity of 
the trachea. In this situation it communicates with the cavity of the 
pharynx above and the cavity of the trachea below. From its anatomic 
relations and its internal structure — the interpolation of the elastic mem- 
branes — the larynx subserves the two widely different yet related func- 
tions, respiration and phonation. 



244 HUMAN PHYSIOLOGY 

The larynx consists primarily of cartilages, the more important of 
which are the thyroid, the cricoid and the arytenoids, united one to another 
in such a manner as to form a more or less rigid framework possessing in 
its different joints a certain amount of motion; secondarily of muscles 
and nerves which conjointly impart to the cartilages the degree of move- 
ment necessary to the performance of the laryngeal functions. The 
larynx is lined throughout by mucous membrane and covered externally 
by fibrous tissue. 

The Vocal Bands. — The mucous membrane, as it passes downward, 
is reflected over the superior thyro- arytenoid ligament, and assists in 
the formation of the false vocal band; it then passes into and lines the 
ventricle, after which it is reflected outward over the superior border of the 
thyro-arytenoid muscle and ligament, and assists in the formation of the 
true vocal band; it then returns upon itself and passes downward over 
the lateral portion of the crico-thyroid membrane into the trachea. 

The thin, free, reduplicated edge of the mucous membrane constitutes 
the true vocal band. The surface of the mucous membrane is covered by 
ciliated epithelium except in the immediate neighborhood of the vocal 
bands. 

The vocal bands are attached anteriorly to the thyroid cartilage near 
the receding angle and posteriorly to the vocal processes of the arytenoid 
cartilages. They vary in length in the male from 20 to 25 mm. and in the 
female from 15 to 20 mm. 

The Muscles of the Larynx. — The muscles which have a direct action 
on the cartilages of the larynx and determine the position of the vocal 
bands both for respiratory and phonatory purposes, and which regulate 
their tension as well, are nine in number and take their names from 
their points of origin and insertion: viz., two posterior crico-arytenoids, 
two lateral crico-arytenoids, two thyro-arytenoids, one arytenoid, and two 
crico-thyroids. 

The posterior crico-arytenoid muscles rotate the arytenoid cartilages 
outward and thus separate the vocal bands and enlarge the aperture of 
the glottis, a condition necessary to the free entrance of the air into the 
lungs. Since the contraction of the crico-arytenoids has this result they 
are frequently spoken of as the abductor or the respiratory muscle. 

The lateral crico-arytenoid muscles are the antagonists of the former. 
Their action is to rotate the arytenoid cartilages inward thus approximat- 
ing the vocal bands. 

The arytenoid muscle consists (1) of transversely arranged fibers which 
arise from and are inserted into the outer surface of the opposite arytenoid 



PHONATION — ARTICULATE SPEECH 245 

cartilages, and (2) of obliquely directed fibers which arise from the outer 
angle of one arytenoid to be inserted into the apex of the other. In their 
course they decussate in the median line. The action of this muscle is to 
approximate the arytenoid cartilages and thus obliterate that portion of 
the glottis between the vocal processes, the rima rcspiratoria. 

The thyro-arytcnoid muscles, acting in conjunction with the lateral 
crico-arytenoids, closely approximate the edges of the vocal bands so 
that the space between them is reduced to a mere slit — the rima vocalis — 
one of the conditions necessary for phonation. 

Collectively these muscles adduct the vocal bands to the middle line 
and thus constrict the glottis. For this reason they are generally spoken 
of as the adductors or the phonatory muscles. 

The crico-thyroid muscle at the time of its contraction draws up the 
anterior part of the cricoid cartilage toward the thyroid, which remains 
stationary, and swings the quadrate plate of the cricoid and the arytenoid 
cartilages downward and backward. This movement has the result of 
tensing the vocal bands. The cricoid is at the same time drawn backward 
by the action of the more longitudinally disposed fibers. 

Movements of the Vocal Bands. — During the intervals of speaking the 
vocal bands are widely separated by the tonic contraction of the poste- 
rior crico-arytenoid muscles. With each inspiration, however, they are 
separated to a somewhat greater extent; with each expiration they return 
to their former condition. 

Phonation. — As soon as phonation is about to take place the vocal 
bands are suddenly approximated, made parallel, and increased in tension. 
When the foregoing conditions in the glottis are realized, the air stored 
or collected in the lungs is forced by the contraction of the expiratory 
muscles, through the narrow space between the bands. As a result of the 
resistance offered by this narrow outlet and the force of the expiratory 
muscles, the air within the lungs and trachea is subjected to pressure, and 
as soon as the pressure attains a certain level the vocal bands are thrown 
into vibrations, which in turn impart to the column of air in the upper 
air-passages a corresponding series of vibrations by which the laryngeal 
vibrations are reinforced. 

The Characteristics of the Vocal Sounds. — All vocal sounds are charac- 
terized by intensity, pitch and quality. 

The intensity or Lou sound depends on the extent or amplitude 

of the tO-and-fro vibration, or the extent of the excursion of the vocal 
band on either side of the position of equilibrium or rest; and this in turn 
depends on th< r h which the blast of air strikes the band. 



246 HUMAN PHYSIOLOGY 

The pitch of the voice depends on the number of vibrations in a unit 
of time, a second. This will be conditioned by the length of the bands in 
vibration or the length and width of the aperture through which the air 
passes and the degree of tension to which the bands are subjected. In 
the emission of sounds of highest pitch the tension of the vocal bands 
and the narrowing of the glottis attain their maximum. In the emission 
of sounds of lowest pitch the reverse conditions obtain. In passing from 
the lowest to the highest pitched sounds in the range of the voice peculiar 
to any one individual, there is a progressive increase in both the tension 
of the vocal bands and the narrowing of the glottic aperture. 

The quality of the voice, the timbre or tone-color, depends on the form 
combined with the intensity and pitch of the vibration. As with sounds 
produced by musical instruments, the primary or fundamental vibration 
of the vocal band is complicated by the superposition of secondary or 
partial vibrations (overtones). The form of the vibration will, therefore, 
be a resultant of the blending of a number of different vibrations. The 
quality of the sound produced in the larynx is, however, modified by the 
resonance of the mouth and nasal cavities; certain of the overtones being 
reinforced by changes in the shape of the mouth cavity more especially, 
thus giving to the voice a somewhat different quality. 



Speech is the expression of ideas by means of articulate sounds. These 
sounds may be divided into vowel and consonant sounds. 

The vowel sounds, a, e, i, 0, u, are Laryngeal tones modified by the 
superposition and reinforcement of certain overtones developed in the 
mouth and pharynx by changes in their shapes. The number of vibra- 
tions underlying the production of each vowel sound is a matter of 
dispute. 

Consonant sounds are produced by the more or less complete interrup- 
tion of the vowel sounds during their passage through the organs of 
speech. These may be divided into : 



1. Labials, p, b, m. 

2. Labio-dentals, /, v. 

3. Linguo-dentals, s, z. 

4. Anterior linguo-palatals, t, d, I, n, r, sh, zh. 

5. Posterior linguo-palatals, k, g> h t y. 






The names of these different groups of consonants indicate the region 
of the mouth in which they are produced and the means by which the air 
blast is interrupted. 



PH0NAT10N — ARTICULATE SPEECH 247 

The Nerves of the Larynx. — The two antagonistic groups of laryngeal 
muscles — the respiratory and the phonatory — are innervated by two dif- 
ferent groups of nerve-fibers both of which however are contained in the 
trunk of the inferior laryngeal nerve. These two groups of nerve-fibers 
have their origin in two separate centers in the floor of the fourth ventricle 
of the medulla. These centers are known as the laryngeal respiratory 
and the phonatory centers. The phonatory center in the medulla is in 
relation with a volitional or motor center in the lower portion of the 
precentral convolution near the anterior border. Stimulation of this area 
is invariably followed by bilateral adduction of the vocal bands and 
closure of the glottis. 



REPRODUCTION 

Reproduction is the function by which the species is preserved; it is 
accomplished by the organs of generation in the two sexes. Embryology 
is the science which investigates the successive stages in the development 
of the embryo. 

GENERATIVE ORGANS OF THE FEMALE 

The generative organs of the female consist of the ovaries, Fallopian 
tubes, uterus, and vagina. 

The ovaries are two small, flattened bodies, measuring about 40 mm. 
in length and 20 mm. in width; they are situated in the cavity of the 
pelvis, and are imbedded in the posterior layer of the broad ligament; 
attached to the uterus by a round ligament, and to the extremities of the 
Fallopian tubes by the fimbriae. The ovary consists of an external mem- 
brane of fibrous tissue, the cortical portion, in which are embedded the 
Graafian vesicles, and an internal portion, the stroma, containing 
blood-vessels. 

The Graafian vesicles are exceedingly numerous, but are situated only 
in the cortical portion. It is estimated that each ovary contains from 
20,000 to 40,000 follicles. Although the ovary contains the vesicles from 
the period of birth, it is only at puberty that they attain their full develop- 
ment. From this time onward to the catamenial period there is a constant 
growth and maturation of the Graafian vesicles. They consist of an 
external investment, composed of fibrous tissues and blood-vessels, in 
the interior of which is a layer of cells forming the membrana granulosa; 
at its lower portion there is an accumulation of cells, the proligerous disc, 
in which the ovum is contained. The cavity of the vesicle contains a 
slightly yellowish alkaline, albuminous fluid. 

The ovum is a globular body, measuring about 0.3 mm. in diame- 
ter. It consists of a mass of protoplasmic material cytoplasm, a nu- 
cleus or germinal vesicle and a nucleolus or germinal spot. The peripheral 
portion of the cytoplasm is surrounded by a clear thick membrane, the 
zona pellucida, external to which is a layer of radially placed columnar, 
epithelium forming the corona radiata. The nucleus consists of a nuclear 
membrane enclosing material, some of which arranged in the form of 
thread stains readily and hence known as chromatin, in the meshes of 
which lies a material that stains faintly and hence known as achromatin. 

The Fallopian tubes are about 1 2 centimeters in length, and extend 
outward from the upper angles of the uterus, between the folds of the 

248 



REPRODUCTION 249 

broad ligaments, and terminate in a fringed extremity which is attached 
by one of the fringes to the ovary. They consist of three coats: 

1. The external, or peritoneal. 

2. Middle, or muscular, the fibers of which are arranged in a circular 
and longitudinal direction. 

3. Internal, or mucous, usually folded longitudinally, is covered with 
ciliated epithelial cells, which are always waving from the ovary toward 
the uterus. 

The uterus is pyriform in shape, and may be divided into a body and 
neck; it measures about 7 cm. in length and 5 cm. in breadth in the 
unimpregnated state. At the lower extremity of the neck is the os 
externum; at the junction of the neck with the body is a constriction, 
the os internum. The cavity of the uterus is triangular in shape, the 
walls of the triangle being almost in contact. 

The walls of the uterus are made up of many layers of non-striated 
muscle-fibers, covered externally by peritoneum, and lined internally by 
mucous membrane, containing numerous tubular glands, and covered by 
ciliated epithelial cells. 

The vagina is a membranous canal, from 12 to 18 cm. in length, 
situated between the rectum and bladder. It extends obliquely upward 
from the surface, almost to the brim of the pelvis, and embraces at its 
upper extremity the neck of the uterus. 

Discharge of the Ovum. — As the Graafian vesicle matures it increases 
in size, from an augmentation of its liquid contents, and approaches the 
surface of the ovary, where it forms a projection, measuring from six to 
twelve cm. The maturation of the vesicle occurs periodically, about 
every twenty-eight days, and is attended by the phenomena of menstrua- 
tion. During this period of active congestion of the reproductive organs 
the Graafian vesicle ruptures, the ovum and liquid contents escape, and 
are caught by the fimbriated extremity of the Fallopian tube, which has 
adapted itself to the posterior surface of the ovary. The passage of the 
ovum through the Fallopian tube into the uterus occupies from ten to 
fourteen days, and is accomplished by muscular contraction and by the 
action of the ciliated epithelium. 

Menstruation is a periodic discharge of blood from the mucous mem- 
brane of the uterus, due to a fatty defeneration of the small blood-vessels. 
Under the pressure of an increased amount of blood in the reproductive 
organs, attending tin- pi ovulation, the blood-vessels rupture, and 

a hemorrhage takes place into the uterine cavity; thence it passes into 



250 



HUMAN PHYSIOLOGY 



the vagina. Menstruation lasts from five to six days, and the amount 
of blood discharged averages from 180 to 200 c.c. 

Corpus Luteum. — For some time previous to the rupture of a Graafian 
vesicle it increases in size and becomes vascular; its walls become thickened 
from the deposition of a reddish-yellow, glutinous substance, a product 
of cell growth from the proper coat of the follicle and the membrana 
granulosa. After the ovum escapes there is usually a small effusion of 
blood into the cavity of the follicle, which soon coagulates, loses its color- 
ing-matter, and acquires the characteristics of fibrin, but it takes no part 
in the formation of the corpus luteum. The walls of the follicle become 
convoluted and vascular and undergo hypertrophy, until they occupy the 
whole of the follicular cavity. At its period of fullest development the 
corpus luteum measures 20 mm. and 12 mm. in depth. In a few weeks 
the mass loses its red color and becomes yellow, constituting the corpus 
luteum , or yellow body. It then begins to retract and becomes pale; and 
at the end of two months nothing remains but a small cicatrix upon the 
surface of the ovary. Such are the changes in the follicle if the ovum 
has not been impregnated. 

The corpus luteum, after impregnation has taken place, undergoes a 
much slower development, becomes larger, and continues during the entire 
period of gestation. The difference between the corpus luteum of the 
unimpregnated and pregnant condition is expressed in the following table 
by Dal ton: 

Corpus Luteum of Menstruation. Corpus Luteum of Pregnancy 



At the end of three 

weeks. 
One month 



Two months. 



Four months. 



Six months.. 



Nine months. 



20 mm. in diameter; central clot reddish; convoluted wall 



pale. 

Smaller; convoluted 
wall bright yellow; 
clot still reddish. 

Reduced to the con- 
dition of an insignifi- 
cant cicatrix. 

Absent or unno- 
ticeable. 



Absent., 



Absent. 



Larger; convoluted wall bright 
yellow; clot still reddish. 

20 mm. in diameter; convoluted 
wall bright yellow; clot perfectly 
decolorized. 

20 mm. in diameter; clot pale 
and fibrinous; convoluted wall dull 
yellow. 

Still as large as at the end of 
second month; clot fibrinous; 
convoluted wall^paler. 

12 mm. in diameter; central clot 
converted into a radiating cicatrix; 
external^ywall tolerably thick and 
convoluted, but without any bright 
yellow color. 



REPRODUCTION 251 

GENERATIVE ORGANS OF THE MALE 

The generative organs of the male consists of the testicles, vasa 
deferentia, vesiculae seminales, and penis. 

The testicles, the essential organs of reproduction in the male, are two 
oblong glands, about 40 mm. in length, compressed from side to side, 
and situated in the cavity of the scrotum. 

The proper coat of the testicles, the tunica albuginea, is a white, fibrous 
structure, about one mm. in thickness; after enveloping the testicle, it is 
reflected into its interior at the posterior border, and forms a vertical pro- 
cess, the mediastinum testis, from which septa are given off, dividing the 
testicle into lobules. 

The substance of the testicle is made up of the seminiferous tubules, 
which exist to the number of 840; they are exceedingly convoluted, and 
when unravelled are about 30 cm. in length. As they pass toward the 
apices of the lobules, they become less convoluted, and terminate in 
from twenty to thirty straight ducts, the vasa recta, which pass upward 
through the mediastinum and constitute the rete testis. At the upper part 
of the mediastinum the lobules unite to form from nine to thirty small 
ducts, the vasa ejferentia, which become convoluted and form the globus 
major of the epididymis; the continuation of the tubes downward behind 
the testicle and a second convolution constitutes the body and globus 
minor. 

The seminal tubule consists of a basement membrane lined by granular 

nucleated epithelium. 

# 
The vas deferens, the excretory duct of the testicle, is about 60 cm. 

in length, and may be traced upward from the epididymis to the under 

surface of the base of the bladder, where it unites with the duct of the 

vesicula seminalis to form the ejaculatory duct. 

The vesiculae seminales are two lobulated, pyriform bodies about 50 
mm. in length, situated on the inner surface of the bladder. 

They have an external fibrous coat, a middle muscular coat, and an 
internal mucous coat, covered by epithelium, which secretes a mucous 
fluid. The vesicula} seminales serve as reservoirs, in which the seminal 
fluid is temporarily stored up. 

The ejaculatory duct, about 20 mm. in length, opens into the urethra, 
and is formed by the union of the vasa deferentia and the ducts of the 
vesiculae seminales. 

The prostate gland surrounds the posterior extremity of the urethra, 
and opens into it by from twenty to thirty openings, the orifices of the 



252 HUMAN PHYSIOLOGY 

prostatic tubules. The gland secretes a fluid which forms part of the semen 
and assists in maintaining the vitality of the spermatozoa: 

The semen is a complex fluid, made up of the secretions from the 
testicles, the vesiculae seminales, the prostatic and urethral glands.* It 
is grayish- white in color, mucilaginous in consistence, of a characteristic 
odor, and somewhat heavier than water. From one to five c.c. is 
ejaculated at each orgasm. 

The spermatozoa are peculiar anatomic elements, developed within the 
seminal tubules, and possess the power of spontaneous movement. The 
spermatozoa consist of a conoid head and a long, filamentous tail, which 
is in continuous and active motion; so long as they remain in the vas 
deferens they are quiescent, but when free to move in the fluid of the 
vesiculae seminales, they become very active. 

Origin. — The spermatozoa appear at the age of puberty, and are then 
constantly formed until an advanced age. They are developed from the 
nuclei of large, round cells contained in the anterior of the seminal tubules, 
as many as fifteen to twenty developing in a single cell. 

When the spermatozoa are introduced into the vagina, they pass readily 
into the uterus and through the Fallopian tubes toward the ovaries, 
where they remain and retain their vitality for a period of from eight to 
ten days. 

Fecundation is the union of the spermatozoa with the ovum during its 
passage toward the uterus and usually takes place in the Fallopian tube 
just outside the uterus* After floating around the ovum in an active 
manner, a single spermatozoan penetrates the ovum, this accomplished, 
the head and body meet and unite with the nucleus of the ovum. A series 
of histologic changes now arise which eventuate in the production of a 
new cell, the parent cell, from which the new being develops through 
successive division, multiplication and differentiation of cells. 

The Fixation of the Ovum. — The ovum after fertilization in the oviduct, 
continues to divide and pass slowly to the uterus (8-10 days) where it is 
retained until the end of gestation. A menstrual mucosa having de- 
veloped, the ovum lodges on a smooth thick area and gradually sinks 
beneath the surface. During the passage down the oviduct the zona 
pellucida has become attenuated and has been finally replaced by a thick 
layer of ameboid and phagocytic cells called the trophoderm. Upon 
lodgment of the ovum these cells destroy the underlying mucosa and pro- 
duce a cavity into which the ovum sinks. As the ovum increases in 
size the mucosa gradually covers it; that portion of the mucosa toward 



REPRODUCTION 253 

the uterine cavity is called the decidua capsularis or rejlexa, that beneath 
the ovum the decidua basilaris or placentalis, while the remainder consti- 
tutes the decidua parietal is or vera. As development proceeds the decidua 
basilaris becomes greater and ultimately develops into the placenta, the 
organ of nutrition and respiration. 

Segmentation of the Ovum. — Immediately after fertilization the ovum 
divides and redivides within the diminishing zona pellucida, forming an 
irregular mass of cells called the morula. The peripheral cells form a layer, 
the trophoderm, beneath the attenuated zona pellucida, ultimately replacing 
that structure. The remaining cells of the morula differentiate into three 
masses, ectodermal, entoderm al and mesodermal. The central cells of these 
masses liquefy and disappear forming thus the ectodermal or amniotic 
cavity, limited by the ectoderm; the entodermal cavity limited by the 
entoderm; and the mesodermal or celomic cavity limited by the extra-em- 
bryonic mesoderm. Meanwhile cells in various parts of the thickened 
trophoderm have disappeared, leaving this layer in the form of delicate 
trophodermal villi, the future chorionic and placental villi. 

The Embryonic Shield. — The floor of the amniotic cavity consisting of 
ectoderm and entoderm constitute the embryonic shield or disk. As the 
shield increases in size, a median longitudinal thickening is seen occupy- 
ing the caudal half of the area. This is the primitive streak, a temporary 
structure that is soon overshadowed by changes in the area just in front 
of it. Here is formed a median longitudinal, grooved ridge of ectoderm 
that develops rapidly in length. This is the neural groove and folds. 
The dorsal lips of the groove approach each other in the mid-line and fuse, 
separating from the original ectoderm which closes over the ectodermal 
tube. This tube is the neural tube from which the nerve system is de- 
veloped. In the immediate vicinity of the head end of the primitive 
streak is seen a darkened area, He?isen's node, that represents the beginning 
invagination of the ectoderm in the formation of the embryonic meso- 
derm and notochord to be considered later. That portion of the em- 
bryonic shield that gives rise to the embryo itself becomes distinctly 
outlined laterally and in the head and tail regions of the neural groove 
Just external to this area, the embryonic area proper, is a transparent area, 
the area pellucida, beyond which is the area opaca in which the first blood- 
vessels appear. 

Mesoderm and Notochord. — So far in the embryonic area only ecto- 
derm and entoderm exist. Hensen's node, at the head end of the primi- 
tive streak, represents an invagination (gastrulation) of ectoderm between 
ectoderm and entoderm. This invagination elongates headward in the 



254 HUMAN PHYSIOLOGY 

embryonic area constituting a tube of ectodermal cells, the chordal canal. 
Later the ventral wall of the canal and the adjacent entoderm disappear, 
so that the chordal ectoderm temporarily forms the dorsal median 
boundary of the entodermal cavity. By this process a communication 
is established between the entodermal cavity and neural groove, called 
the neuro-enteric canal. The chordal ectoderm separates from the ento- 
derm and then forms a solid cord of cells, the notochord; between entoderm 
and neural groove the neurenteric canal, however, persisting for some 
time. In the meanwhile, other ectodermic cell's in the region of the 
chordal invagination Spread between ectoderm and entoderm and form 
the anlage of the mesoderm. These cells by rapid proliferation soon 
separate ectoderm and entoderm and join the extra- embryonic mesoderm. 
The separation of these two structures is complete except in the regions 
of the bucco- pharyngeal and cloacal membranes. 

On each side of the neural groove the mesoderm becomes transversely 
grooved in its ectodermal surface forming a number of successive block- 
like masses called primitive somites or segments; of these, there are thirty- 
eight for the trunk and possibly four for the head regions. Each segment 
consists, of three parts, the sclerotome, the myotome and the dermatome. 
Lateral to the somite is a thickened mass of mesoderm, the intermediate- 
cell mass, that laterally divides into two layers; the outer accompanies the 
ectoderm forming the somatopleure, which gives rise to the body wall; 
the inner joins the entoderm, forming the splanchnopleure from which 
the gut tract, vitelline duct and yolksac are derived. 

Fetal Membranes. — As the primitive streak and neural groove are 
forming, the extra-embryonic mesoderm that lies beneath the tropho- 
derm invades the trophodermic villi, forming there the chorion with its 
villi. Gradually the mesoderm of the roof of the amniotic cavity divides 
into two layers, the upper constituting chorionic mesoderm, while the 
under one is attached to the ectoderm of the amniotic, and forms with the 
latter, the Amnion. In the chick and some mammals the amnion is 
derived from the somatopleure in the folding of! of the body. In amniotes 
the amniotic cavity is at first small, but rapidly increases in size. It. 
contains a clear fluid, the amniotic fluid, which amounts at term to about 
one quart. It serves to protect the fetus during gestation, and at par- 
turition it dilates the os cervis and flushes the birth canal. This liquid 
is derived mainly from the blood as it contains albumin, sugar, fat and 
inorganic salts. Traces of urea indicate that some of its constituents are 
derived from the embryo itself. 

The caudal end of the embryonic area is left connected with the chorion 
by a heavy band of mesoderm termed the belly-stalk to which the caudal 



REPRODUCTION 255 

part of the amnion is attached. The entoderm is invaginated into the 
belly-stalk for a short distance constituting the allantois of higher forms; 
the allantois grows out between the closing somatopleure folds forming 
the body wall and constitutes a free sac upon which vessels, allantoic 
arteries and veins, develop from the embryo. This sac then spreads 
beneath the white shell membrane forming the organ for nutrition and 
respiration of these forms during the last half of their incubation periods. 
In mammals the extra-embryonic portion of the allantois is of little 
importance. 

The Formation of the Placenta. — The chorionic villi increase rapidly 
in size and number and usually surround the whole fetal sac giving it a 
peculiar shaggy appearance. Blood-vessels now proceed from the embryo 
along the belly-stalk (not the allantois in higher forms as formerly stated). 
There the umbilical arteries and veins pass to the chorionic villi and send 
branches of those of the placental area; these vascularized villi constitute 
the chorion frondosum, while the avascular villi form the chorion leva. The 
villi of the latter disappear during the second month, leaving the chorionic 
membrane smooth. The villi of the chorion frondosum now penetrate the 
uterine glands of the decidua basilaris which by this time have been de- 
nuded of epithelium and have gained connection with the blood-vessels of 
the mucosa; in this manner these uterine glands have become converted 
into blood sinuses. The chorionic villi either attach themselves to the 
tunica propria of the mucosa (fixed villi) or remain free, floating villi. At 
the edge of the placental area very few villi develop leaving a circular 
channel called the marginal sinus. This attachment of the villi becomes 
marked from the third month on and is considered the beginning of placen- 
tation. From this time on to full term there is merely an increase in 
number of villi and vessels and thus an increase in the size of the placenta. 

The placenta is the most important of the fetal structures. As it 
develops, conditions are established which permit of a free exchange of 
material between mother and child. Whether by osmosis or by an act of 
secretion, the nutritive materials of the maternal blood pass through the 
intervening membrane into the fetal blood on the one hand, while waste 
products pass in the reverse direction into the maternal blood on the other 
hand. Inasmuch as oxygen is absorbed and carbon dioxid exhaled by the 
same structures, the placenta is to be regarded as both a digestive and a 
respiratory organ. So long as these exchanges are permitted to take 
place in a normal manner the nutrition of the embryo is secured. 

The Nutrition of the Embryo. — As the ovum passes down the oviduct 
it imbibes nutritive materials from the mucosa. As it lodges in the uterus 



256 HUMAN PHYSIOLOGY 

it is nourished at first in the same way. The first circulation developed is 
the vitelline, but as the amount of nutritive material is very small in mam- 
mals its activity is limited. In the oviparous forms, however, where the 
nutritive material is large in amount this circulation is important. The 
allantoic circulation is likewise of importance in the oviparous forms and 
constitutes their last fetal circulation. In mammals the allantoic circula- 
tion is merely a transitional stage in the formation of the placental 
circulation. 

Circulation of Blood in the Fetus. — The blood returning from the pla- 
centa, after having received oxygen and being freed from carbonic acid, 
is carried by the umbilical vein to the under surface of the liver; here a por- 
tion of it, about one-half, passes through the ductus venosus into the ascend- 
ing vena cava, while the remainder flows through the liver and passes into 
the inferior vena cava by the hepatic veins. When the blood is emptied 
into the right auricle, it is directed by the Eustachian valve through the 
foramen ovale, into the left auricle, thence into the left ventricle, and so 
into the aorta and to all parts of the system. The venous blood returning 
from the head and upper extremities is emptied, by the superior vena cava, 
into the right auricle, from which it passes into the right ventricle, and 
thence into the pulmonary artery. Owing to the condition of the lung 
only a small portion flows through the pulmonary capillaries, the greater 
part passing through the ductus arteriosus, which opens into the aorta at 
a point below the origin of the carotid and subclavian arteries. The 
mixed blood now passes down the aorta to supply the lower extremities, 
but a portion of it is directed, by the hypogastric arteries, to the placenta, 
to be again oxygenated. 

At birth, the placental circulation gives way to the circulation of the 
adult. As soon as the child begins to breathe, the lungs expand, blood 
flows freely through the pulmonary capillaries, and the ductus arteriosus 
begins to contract. The foramen ovale closes about the tenth day. The 
umbilical vein, the ductus venosus, and the hypogastric arteries become 
impervious in several days as far as the bladder. Their distal ends ulti- 
mately form rounded cords. 

Physiologic Activities of the Embryo. — During intrauterine life the 
evolution of structure is accompanied by an evolution of function. The 
relatively simple and uniform metabolism of the undifferentiated blasto- 
dermic membranes gradually increases in complexity and variety, as the 
individual tissues and organs make their appearance and assume even a 
slight degree of functional activity. As to the periods at which different 
organs begin to functionate, but little is positively known. 



REPRODUCTION 257 

The primitive heart, in all probability, begins to pulsate very early, as 
in an embryo from fifteen to eighteen days old and measuring but 2.2 mm. 
in length, Coste found the amnion, the allantois, the omphalo-mesenteric 
vessels, and the two primitive aortse developed. In the earlier weeks, 
all products of metabolism are doubtless eliminated by the placental 
structures; but as metabolism increases in complexity the liver and kidney 
assume excretory activity. Thus, at the end of the third month the intes- 
tine contains a dark, greenish, viscid material — meconium — composed of 
bile pigments, bile salts, and desquamated epithelium; the amniotic fluid, 
as well as the fluid within the bladder, contains urea at the end of the sixth 
month, indicating the establishment of both hepatic and renal activity. 
Contractions of the skeletal muscles of the limbs begin about the fifth 
month, from which it may be inferred that the mechanism for muscle 
activity, viz., muscles, efferent nerves, and spinal centers, has become 
anatomically developed and associated, and capable of coordinate activity. 
These contractions are, in all probability, automatic or autochthonic in 
character due to stimuli arising within the spinal centers. The remaining 
organs remain more or less inactive. 

After birth, with the first inspiration and introduction of food into the 
alimentary canal, the physiologic mechanisms which subserve general 
metabolism begin to functionate and in the course of a week are fully 
established. At this time the cardiac pulsation averages about 135 a 
minute; the respiratory movements vary from 30 to 35 a minute, and are 
diaphragmatic in type; the urine, which was at first scanty, is now abun- 
dant and proportional to the food consumed; the digestive glands are 
elaborating their respective enzymes, digestion proceeding as in the adult. 
The hepatic secretion is active and the lower bowel is emptied of its con- 
tents; the coordinate activities of the nerve-, muscle-, and gland-mechan- 
isms are entirely reflex in character. Psychic activities are in abeyance 
by reason of the incomplete development of the cerebral mechanisms. 



17 



INDEX 



ABDUCENT NERVE, 209 
Absorption, 86 

by blood-vessels, 92 

by lacteals, 92 

of oxygen in respiration, 130 
Accommodation of the eye, 230 
Adrenal bodies, 165 
Adult circulation, establishment of, at 

birth, 236 
Air, atmospheric, composition of, 129 

amount exchanged in respiration, 
128 

changes in, during respiration, 130 
Alcohol, action of, 58 
Alimentary principles, classification of, 
55 

carbohydrate principles, 56 

fat principles, 56 

inorganic principles, 57 

protein principles, 55 
Amino-acids, 78, 79 
Amnion, formation of, 254 
Animal heat, 135 

Anterior columns of spinal cord, 172 
Aphasia, 199 
Area, germinal, 239 
Arteries, properties of, 114 
Articulations, or joints, 11 

classification of, 11 
Astigmatism, 233 
Autonomic nerve system, 202 

functions of, 205 

BILE, 80 

mode of secretion, 82 

physiologic action, 83 
Bladder, urinary, 148 
Blastodermic membranes, 253 
Blood, 94 

changes in, during respiration, 131 

circulation of, 102 

coagulation of, 96 

coloring-matter of, 95. 98 

composition of, plasma, 97 

corpuscles, 98 

gases of. 131 

origin of, 99 

pressure, 116 

rapidity of flow in arteries, 119 

rapidity of flow in capillaries, 119 
Bronchial innervation, 124 
Burdach, column of, 173 



CAPILLARY BLOOD-VESSELS, 

115 

Capsule, internal. 186 

external, 186 



114, 



Cardiac cycle, 107 
Caudate nucleus, 186 
Cells, structure of, 5 

manifestations of life by, 7 

reproduction of, 9 
Center for articulate language, 199 
Central organs of the nerve system and 

their nerves, 169 
Cerebellum, 200 

forced movements, 202 
Cerebrum, 188 

fissures and convolutions, 189 

functions of, 191 

localization of functions, 193 

motor area of, 197 
speech area, 199 
writing area, 199 

sensor areas of, 193 
Chorda tympani nerve, course and func- 
tion of, 212, 213 
Chorion, 255 
Chyle, 93 

Ciliary muscle, 225, 230 
Circulation of blood, 102 
Claustrum, 186 
Cochlea, 241 

Columns of spinal cord, 172 
Corium, 149 
Corpora quadrigemina, 
Corpus luteum, 250 

striatum, 185 
Corti, organ of, 229 
Cranial nerves, 206 
Crura cerebri, 183 
Crystalline lens, 226 

DEGLUTITION, 65 
Digestion, 61 
Ductus arteriosus, 256 
venosus, 250 

EAR, 235 
Electrotonus, 50 
Embryo, activities of, 256 
Embryonic shield, 253 
Endolymph, 242 
Epidermis, 149 
Epididymis, 251 
Eustachian tube, 237, 240 
Excretion, 139 
Eyeball, movements of , 234 
Eye, 223 

blind spot of, 231 

refracting apparatus of, 228 

FACIAL NERVE, 212 

paralysis, symptoms of, 213 



184 



259 



260 



INDEX 



Fallopian tubes, 248 

Feces, 86 

Fermentation, intestinal, 85 

Female organs of generation, 248 

Fetus, circulation of blood in, 256 

Fissures and convolutions of brain, 184 

Foods and dietetics, 52 

animal, 59 

carbohydrate principles of, 55 

cereal, 60 

daily amount required, 53 

energy of, 53, 54 

fat principles of, 55 

inorganic principles of, 55 

percentage composition of, 59 

protein principles of, 55 

vegetable, 61 
Fovea centralis, 225 

GALVANIC CURRENTS, EFFECT 

on nerves, 50 
Ganglia, 40 

Gasserian, 210 

ophthalmic, 203 

semilunar, 204 

spheno-palatine, 212 
Gastric digestion, 67 

glands, 67 

juice, 69 

action of, 71 
Generation, male organs of, 251 

female organs of, 248 
Globules of the blood, 98 
Glomeruli of the kidneys, 145 
Glosso-pharyngeal nerve, 214 
Glottis, respiratory movements of, 130 
Glycogen, 159 

Glycogenic function of the liver, 159 
Goll, column of, 173 
Graafian follicles, 248 

HAIR, 149 

Hearing, sense of, 234 
Heart, 102 

auriculo ventricular bundle, 105 

blood supply, no 

course of blood through, 103 

ganglia of, in 

influence of pneumogastric nerve 
upon, in 

influence of nerve system upon, in 

intraventricular pressure, 109 

Keith Flack node, 106 

sounds of, 108 

valves of, 104 
Heat production, 136 

dissipation, 138 
Hemianopsia, 207 
Hemoglobin, 95. 98 
Hyaloid membrane, 226 
Hypermetropia, 233 
Hypoglossal nerve, 219 
Hypophysis cerebri, 164 

INCUS BONE, 237 
Insalivation, 63 

nerve mechanism of, 64 
Inspiration, movements of thorax in, 126 



Internal capsule, 186 

results of injury to, 188 

secretion, 161 
Intestinal digestion, 74 

juice, physiologic action, 79 
Intra-pulmonic pressure, 125 

thoracic pressure, 125 
Iris, 225 

action of, 231 

KIDNEYS, 143 

formation of urine by, 146 

LABYRINTH OF INTERNAL EAR, 
240 

function of cochlea, 242 

function of semicircular canals, 242 
Larynx, 243 

Lateral columns of spinal cord, 173 
Laws of muscular contraction, 50 
Lens, crystalline, 226 
Levers, 29 
Liver, 157 

conjugation of products of protein 
putrefaction, 160 

formation of urea, 160 

production of glycogen, 159 

secretion of bile by, 158 
Localization -of functions in cerebrum, 

193 
Lungs, 123 

changes in blood while passing 
through, 116 

movements of, 127 

vital capacity of, 129 
Lymph, 90 

corpuscles, 90 
Lymphatic glands, 89 

vessels, origin and course of, 88 

MALLEUS BONE, 237 
Mammary glands, 154 

secretion of milk by, 156 
Mastication, 62 

muscles of, 62 

nerve mechanism of, 62 
Medulla oblongata, 181 

functions of, 182 
Membrana tympani, 235 
Menstruation, 249 
Mesoderm and notocord, 253 
Middle ear, 235 
Milk, 155 

Motor centers of cerebrum, 197 
Muscles, properties of, 17 

changes in, during contraction, 19 

special physiology of, 28 
Muscle-fiber, histology of, 16 
Myopia, 233, 238 

NERVE, OLFACTORY, 206 
abducent, 209 
acoustic, 214 
cells, structure of, 35 
facial, 212 

fibers, structure of, 35 

terminations of, 37 

glosso-pharyngeal, 214 



cf 



_ 

impulse, rate of transmission, 
49 

motor occuli, 208 

optic, 207 

pneumogastric, 215 

roots, function of ventral and dorsal, 
44 

spinal accessory, 217 

tissue, histology of, 35 

trigeminal, 210 

trochlearis, 209 

trunks, structure of, 38 
Nerves, afferent, 43 

classification of, 42 

cranial, 206 

degeneration of, 45 

development and nutrition of, 41 

efferent, 42 

properties^ and functions of, 46 

relation of, to spinal cord, 41 

spinal, 40, 42 

vaso-motor, 121 
Nerve tissue, physiology of, 34 

autonomic, 202 
Neuron, 35 
Nucleus caudatus, 186 

»lenticularis, 186 
OLFACTORY NERVES, 206 
Ophthalmic ganglion, 208 
Optic nerves, 207 
defects, 232 
functions of, 186 
thalamus, 186 
Organs of Corti, 242 
Ovaries, 168, 248 
Ovum, 248 

discharge of, from 
249 
Oxygen, absorption of, by hemoglobin, 
99. 113 



INDEX 



261 



the ovary, 



PANCREAS, 75 
Pancreatic juice, 77 

physiologic action, 78 
Parathyroids, 163 
Peptones, 72 
Perilymph, 241 
Perspiration, 150 
Petrosal nerves, large and small, 212, 

213 
Phonation, 243 
Pituitary, 164 
Physiology, definition of, 2 
Placenta, formation and function of, 

255 
Pleura, 125 

Pneumogastric, vagus nerve, 215 
Pons varolii, 182 
Portal vein, 87 

Posterior columns of spinal cord, 
Prehension, 61 
Presbyopia, 232 

Pressure of blood in arteries, 116, 
Ptyalin, 89 

Pulse, 118 UREA, 140 

Pyramidal tracts, 179 Uric acid, 142 



173 



118 



RED CORPUSCLES OF BLOOD, 98 

chemic composition, 98 

function, 99 

origin of, 99 
Reflex movements of spinal cord, 175 
Reproduction, 248 
Respiration, 122 

chemistry of, 129 

establishment after birth, 134 

movements of, 126 

types of, 127 

nerve mechanism, 133 

volumes of air breathed, 128 

total respiratory exchange, 152 
Retina, 225 
Rigor mortis, 26 

SALIVA, 63 

physiologic action of, 64 

Sebaceous glands, 161 

Semen, 252 

Semicircular canals, 241, 242 

Sight, sense of, 223 

Skeleton, physiology of, 9 

Skin, 148 

Smell, sense of, 222 

Sounds of heart, 108 

Speech, 246 

Spermatozoa, 252 

Spheno-palatine ganglion, 212 

Spinal accessory nerve, 217 
cord, 171 

as an independent center, 173 
function of, as a conductor, 

178 
nerves, origin of, 40, 41 
reflex action of, 175 
segmentation of, 171 
structure of white matter, 172 
structure of gray matter, 171 
special centers of, 178 
reflex activity, 174 

Starvation, phenomena of, 58 

Stomach, 67 

movements of, 72 
nerve mechanism of, 73 

Sudoriparous glands, 150 

Supra-renal capsules, 165 

TASTE, SENSE OF, 221 

nerve of, 221 
Teeth, 62 

Tensor tympani muscle, 237, 239 
Testicles, 168, 251 
Thoracic duct, 89 
Thorax, enlargement of, in inspiration, 

126 
Thyroid gland, 161 
Tongue, 222 

motor nerve of, 219 

sensory nerve of, 210 
Touch, sense of, 219 
Trachea, 123 
Trochlearis nerve, 209 
Turck, column of, 172 



262 INDEX 

Urination, mechanism of, 148 Vascular glands, 16 1 

Urine, 139 Vaso-motor nerves, origin of, 121 

average quantity of solids secreted Veins, 115 

daily, 140 Vertebral column, 10 

composition of, 140 Vesiculae seminales, 251 

Uterus, 249 Villi, structure and functions, 92 

Vision, psychic center for, 197 

VAGUS NERVE, 215 Vital capacity of lungs, 129 

relation of to the heart, in Vocal bands, 244 

to respiration, 134 Visual angle, 229 

Vapor, watery, of breath, 130 Vocal sounds, 24s, 248 



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